I thought I understood the question until the last statement. Sine waves are devoid of harmonics. The use of the word “brightness” generally, denotes more harmonics... more harmonic content. The only Waveform you cannot use to describe ‘brightness’ and ‘dullness’ would be the Sine wave.

So this leads me to believe either your issue lies somewhere else (your Sound System perhaps) or your expectations.

Let’s start by saying: please use any other Waveform to judge brightness/dullness... any other Waveform, but the Sine.

In general, a Sawtooth Waveform, defined as a Waveform containing every whole integer multiple of the fundamental (every harmonic), is rich in harmonics and makes a good source wave to judge brightness across the harmonic spectrum. FM-X like most Frequency Modulation synths utilizes a Level Scaling Function which allows you to control the Modulation Index (the level of the signal causing the harmonic sidebands that are being created). (Described below)...

But before we get there, if you are feeling a dullness in the low end (assuming it is not an ear/hearing issue) we have to ask, what kind of sound system are we talking about?

How we hear:
In general, a Sine wave at 1kHz Set so it moves a VU meter to +0VU, will naturally sound significantly louder to your ears than a Sine wave at 100Hz registering +0VU. Even though they are doing exactly the same work, the 1kHz wave will sound significantly louder to you. The same Sine wave at 10kHz doing +0VU will also sound significantly quieter to your ear (thankfully).

This proves that we, as humans, have a natural frequency response curve that allows us to hear best in the Mid-Range. The lower the pitch, the softer it is, the higher the pitch the softer it is... we hear best in the middle. This is why on home hifi's you have a BASS and TREBLE control because you don't hear them so well!

I mention that because it’s important to recognize that we do have a natural Frequency Response Curve (and it ain’t flat)... expect to hear differently across the frequency spectrum.
Also: Expect no harmonics from Sine waves.

In a Modulator-Carrier relationship - the more you increase the output level of the Modulator(s) the more sidebands/harmonics are produced via the Carrier. You can 'scale' the response of this influence so that one area of the keyboard gets more than another. This is handled in the FM-X programming - described below...

Carriers Are Dull:
You mention Carriers getting dull.... to reverse that, you can use the Level Scaling to increase the Modulator Index as you go lower across the scale...

In a simple two Operator stack (example: where Operator 1 is Modulator and Operator 2 is Carrier... you would edit the “Lvl/Key Lo” and the “Curve Lo” parameters of Operator 1... and the Break Point - the fulcrum point across the keyboard (typically C3).
On your FM sound, Select the FM-X PART, Press [EDIT] > the Operators are accessed across the bottom of the touch screen...

The way this Scaling works can be explained by the following diagram:

The keyboard is divided in two at the break point.
The high-pitch side at the right is set by using the R Depth and the R Curve; the low-pitch side at the left is set by using the L Depth and the L Curve as described below:

The Output level of the Key set as the Break Point depends on the Operator Level setting. For the Keys on the left side of the Break Point, the Output level is adjusted based on the curve which is determined by the Low Curve and Low Depth.

The Output level changes in an exponential fashion from the Break Point on the Exp type curve and the Output level changes in a linear fashion from the Break Point in the Linear type curve. In either case, the farther away from the Break Point the key is, the greater the Output level changes for the key.

Basic FM Synthesis:
In general, you can have the Modulators influence increase as you go lower, if you set the "Curve Lo" to +Linear and raise the Lvl/Key Lo to your taste.
"+Linear" is a more direct (steep) change than the "+Exponential". The "Break Point" is the KEY along the scale at which this "see-saw" balances left from right.

You can understand the graph when it is a line going straight across... If it is a flat line, this would represent all keys with the same output level.
A Negative Linear to the left of the Break Point means the sound will get rounder as you get lower.
A Positive Linear Curve (+Linear) to the left of the Break Point means the sound will not get rounder as you go lower... it will actually have more 'brightness'.
To the right of the Break Point, you can prevent a M:C relationship from producing those wild HIGH FREQ squeaks by applying a Negative Curve.

In fundamental FM: the more energy to to the Modulator the more harmonic activity produced by the associated Carrier... this is why we are increasing harmonic content to get brighter (no modulator influence, you'd have a Carrier which, if set to a Sine, would have no harmonics).
Make sense? Try it...

Hope that helps. Let us know.

I thought I understood the question until the last statement. Sine waves are devoid of harmonics. The use of the word “brightness” generally, denotes more harmonics... more harmonic content. The only Waveform you cannot use to describe ‘brightness’ and ‘dullness’ would be the Sine wave.

So this leads me to believe either your issue lies somewhere else (your Sound System perhaps) or your expectations.

Let’s start by saying: please use any other Waveform to judge brightness/dullness... any other Waveform, but the Sine.

In general, a Sawtooth Waveform, defined as a Waveform containing every whole integer multiple of the fundamental (every harmonic), is rich in harmonics and makes a good source wave to judge brightness across the harmonic spectrum. FM-X like most Frequency Modulation synths utilizes a Level Scaling Function which allows you to control the Modulation Index (the level of the signal causing the harmonic sidebands that are being created). (Described below)...

But before we get there, if you are feeling a dullness in the low end (assuming it is not an ear/hearing issue) we have to ask, what kind of sound system are we talking about?

How we hear:
In general, a Sine wave at 1kHz Set so it moves a VU meter to +0VU, will naturally sound significantly louder to your ears than a Sine wave at 100Hz registering +0VU. Even though they are doing exactly the same work, the 1kHz wave will sound significantly louder to you. The same Sine wave at 10kHz doing +0VU will also sound significantly quieter to your ear (thankfully).

This proves that we, as humans, have a natural frequency response curve that allows us to hear best in the Mid-Range. The lower the pitch, the softer it is, the higher the pitch the softer it is... we hear best in the middle. This is why on home hifi's you have a BASS and TREBLE control because you don't hear them so well!

I mention that because it’s important to recognize that we do have a natural Frequency Response Curve (and it ain’t flat)... expect to hear differently across the frequency spectrum.
Also: Expect no harmonics from Sine waves.

In a Modulator-Carrier relationship - the more you increase the output level of the Modulator(s) the more sidebands/harmonics are produced via the Carrier. You can 'scale' the response of this influence so that one area of the keyboard gets more than another. This is handled in the FM-X programming - described below...

Carriers Are Dull:
You mention Carriers getting dull.... to reverse that, you can use the Level Scaling to increase the Modulator Index as you go lower across the scale...

In a simple two Operator stack (example: where Operator 1 is Modulator and Operator 2 is Carrier... you would edit the “Lvl/Key Lo” and the “Curve Lo” parameters of Operator 1... and the Break Point - the fulcrum point across the keyboard (typically C3).
On your FM sound, Select the FM-X PART, Press [EDIT] > the Operators are accessed across the bottom of the touch screen...

The way this Scaling works can be explained by the following diagram:

The keyboard is divided in two at the break point.
The high-pitch side at the right is set by using the R Depth and the R Curve; the low-pitch side at the left is set by using the L Depth and the L Curve as described below:

The Output level of the Key set as the Break Point depends on the Operator Level setting. For the Keys on the left side of the Break Point, the Output level is adjusted based on the curve which is determined by the Low Curve and Low Depth.

For the Keys on the right side of the Break Point, the Output level is adjusted based on the curve which is determined by the High Curve and High Depth.

The Output level changes in an exponential fashion from the Break Point on the Exp type curve and the Output level changes in a linear fashion from the Break Point in the Linear type curve. In either case, the farther away from the Break Point the key is, the greater the Output level changes for the key.

Basic FM Synthesis:
In general, you can have the Modulators influence increase as you go lower, if you set the "Curve Lo" to +Linear and raise the Lvl/Key Lo to your taste.
"+Linear" is a more direct (steep) change than the "+Exponential". The "Break Point" is the KEY along the scale at which this "see-saw" balances left from right.

You can understand the graph when it is a line going straight across... If it is a flat line, this would represent all keys with the same output level.
A Negative Linear to the left of the Break Point means the sound will get rounder as you get lower.
A Positive Linear Curve (+Linear) to the left of the Break Point means the sound will not get rounder as you go lower... it will actually have more 'brightness'.
To the right of the Break Point, you can prevent a M:C relationship from producing those wild HIGH FREQ squeaks by applying a Negative Curve.

In fundamental FM: the more energy to to the Modulator the more harmonic activity produced by the associated Carrier... this is why we are increasing harmonic content to get brighter (no modulator influence, you'd have a Carrier which, if set to a Sine, would have no harmonics).
Make sense? Try it...

Hope that helps. Let us know.

Excellent. Thanks so much for all the info.
Please ignore the sine wave part of the comment. I now realise that is impossible. ?
I am playing through a turbosound Iq system with two iq15 tops and two iq18 subs in my studio live room so it’s not the playback system.
My OB8 and all my other synths sound just fine.
I have always struggled getting an even tonality across the keyboard with DX style instruments (I just sold my DX5 now I have the MODX and also have a TX816 here).
Great you have pointed out what needs to be done with the modulator part of the algorithm. It’s so much easier editing the FM sounds on this synth compared to my previous DX5. Really looking forward to programming the FM side more anything else on the MODX for the moment.
So great to have instant help available.
Thank you.

Thanks for the question. The degree to which a sound reproduces the higher harmonics has everything to do with what we perceive as its clarity, and we use words like ‘brightness’ to describe. In a subtractive synthesis situation, you use filters to remove high harmonics. With a LPF this is always top down removal of harmonics. In FM synthesis, you ‘grow’ the high harmonics in a different way. The energy and application of the Modulator determines the brightness of the result through the Carrier.

A Modulator can be programmed to increase its output index in response to velocity... this, combined with the scaling, is how you can make a sound ‘behave’ as our ear/brain expects... the louder it gets the more high harmonics we expect to hear, while maintaining the clarity and focus across the scale.

We’ll have a MODX FM-X tutorial coming up soon, look for it in the MODX “LEARN” area in the coming weeks.

Thanks for the question. The degree to which a sound reproduces the higher harmonics has everything to do with what we perceive as its clarity, and we use words like ‘brightness’ to describe. In a subtractive synthesis situation, you use filters to remove high harmonics. With a LPF this is always top down removal of harmonics. In FM synthesis, you ‘grow’ the high harmonics in a different way. The energy and application of the Modulator determines the brightness of the result through the Carrier.

A Modulator can be programmed to increase its output index in response to velocity... this, combined with the scaling, is how you can make a sound ‘behave’ as our ear/brain expects... the louder it gets the more high harmonics we expect to hear, while maintaining the clarity and focus across the scale.

We’ll have a MODX FM-X tutorial coming up soon, look for it in the MODX “LEARN” area in the coming weeks.

Excellent. Thank you so much. Really look forward to seeing the FM-X tutorial. ?
Yes, coming from OB8, Pro 08, Sub37, Taurus 3 programming, the FM-X is certainly different (understatement). I think it can be made to sound very warm though which is something that I never appreciated much back in the late 80’s where FM was predominately used for clanging kind of sounds. I was working on a really nice Oberheim style pad. It was only the evenness of ‘brightness’ across the keys I was struggling with. Thanks.