When you sum a full stereo signal into mono, you degrade the sound. It becomes boxy, the highs and lows lose definition. The amp you are using is mono (not stereo).

With a mono sound system you cannot hear the spaciousness and clarity of the stereo sound. That what you are hearing... the loss of the stereo.

If you must play the instrument in mono, connect only one cable from the "L/Mono" Output jack to one Input of the amp. This will at least maintain the phase coherence of the signal. "L/Mono" is designed to combine both Left and Right signal so that you don't get phase cancellation, and while not the same as glorious stereo, at least you reduce the boxy cancellation.

You should invest in a professional sound system to translate the full stereo MOXF sound. Not only are some of the instruments sampled in stereo (pianos, strings, brass sections, drums, etc) but a lot of the Effect processors utilize stereo. The B3 organs, are sampled in mono, but the Rotary Speaker Effects uses the left/right stereo movement to recreate the Leslie. The Rhodes sounds are sampled in mono, but the Auto Pan effect recreates the Suitcase Rhodes' left/right tremolo.

These things you will not hear in a mono system. A stereo system allows signals to be "moved" across the horizon left-to-right, you can position a sound within the panorama by sending more signal to one channel than to the other. The ear/brain interprets this as the sound's position, instead of coming directly from the speaker, it sounds like it is somewhere in between.

Try the "16+5&1/3+8" B3 organ Voice in headphones, then in you amp... use the MW to bring in the Rotary Speaker
Try the "Vintage'74" Rhodes in headphones, experience the lush movement left/right,... but in your amp it sounds like nervous tremolo - the lush slow movement sounds like a double-time warble.

Also try the mono version of Full Concert Grand, it's called: "Monaural Grand MW" Preset 1:016(A16) - it should translate just fine in both headphones and in your keyboard amp. It's a mono sound!

Options:
_Buy a second keyboard amp, operate in stereo
_Trade it in for an appropriate stereo sound reinforcement system. You're ready, you deserve it, your keyboard synth demands it! With today's powered speaker systems you can get more power, and better sound recreation for a reasonable investment.

Hope that helps. Stereo, it's how you always hear music, life itself is in stereo!

Thanks for the info. Any recommendation for powered speakers/monitor
that are reasonably priced? Thanks.

"Reasonably priced" is too big an unknown factor. Do you mean 'for what you get', or do you have a price limit? See what I mean? There are speakers and there are speakers.... spend too little, expect them to have performance problems... In general, you get what you pay for, there is no magic bullet here.

Some Powered speakers can run hot, this is bad news, if you are driving them up loud for extended periods. Many users discover this only when it's too late.

For not much more money and certainly not much more weight than what you currently have you can get a quality powered speaker system (stereo) that is both full range and high fidelity. Most keyboard amps top out at the 165-185 Watt range, while today's powered sound reinforcement system will deliver over 700 watts of continuous power (more than 1000 Watts peak) per side. Will make any keyboard amp sound tiny in comparison.

Your sound system is an important part of your rig, and worth the investment... and, I hate to say this, but systems get better and better as you throw more money at them... up til you run out of money 🙂

In the Yamaha world... start by looking at the convenient StagePas 600i. Stereo, comes with a 10 channel mixer, lightweight, personal PA system. The easiest to transport, high fidelity system there is 340 Watts per side!

Next up the line are the powered sound reinforcement systems DBR, DXR, and DSR. (Over 1000 Watts per side)
As you increase in price, there's are trade offs... the DSR sound the best (cost the most, naturally) they are wood... heavy, but wood does resonate musical tones (subtle but undeniably true).

The DBR and DXR deliver the sound in lighter weight cabinets - not being wood, they will last longer, and stand for more abuse during load-in and loud-out. I like the DXR (medium price) which come in 8", 10", 12" or 15" sizes. The Yamaha's deliver the sound under all high volume, high stress situations for longer periods of time - something you can only appreciate with time (if you've ever had a powered cabinet shut down on you under long hard use, you know what I mean) ... so-called budget brands, and even some well known cabinets, are guilty here.

Shop around, ask around. Again "reasonable" is relative and really depends a lot on your personal requirements. It is an investment. Being known for great sound will ultimately help you get more gigs and will mean they will pay for themselves quicker, in the long run!

But STEREO, is the key. It's better than mono, by a greater factor than just 2. Life itself, and music in particular, is in STEREO!
🙂

What's the difference between Full Concert Grand and Monaural Grand? They sound quite different. They also have different waveforms assigned (some doubled, some omitted). It doesn't seem to me Monaural Grand is a pure mono version of Full Grand..

I prefer Monaural Grand, sounds more mid-foucesd and vintage to me...

What's the difference between Full Concert Grand and Monaural Grand? They sound quite different. They also have different waveforms assigned (some doubled, some omitted). It doesn't seem to me Monaural Grand is a pure mono version of Full Grand..

They are both programs made from a Yamaha CFIIIS Concert Grand piano... "pure" is the meaningless word in your question. Why does anything have to be "pure"... or better, what does that even mean?

The programmers are not tasked with being "pure". The "Full Concert Grand" is made from stereo samples... play it back in stereo otherwise you cannot truly evaluate it. The "Monaural Grand" is made from mono samples... what the programmers are tasked with doing is to make each sound as good as is possible.

I prefer Monaural Grand, sounds more mid-foucesd and vintage to me...

If mid-focused and vintage is your thing, then this is understandable. And then you begin to understand why Yamaha offers a variety of pianos, and a variety of piano treatments.

You don't mention whether you are listening in stereo or mono. You cannot hear stereo in one speaker, stereo Voices in mono are degraded by the nature of your system. They are severely degraded if you sum the Left and Right outputs to a single speaker. If you must connect in mono, use just the Left/Mono Output jack as it will phase coherently sum the signal...and while this is still not stereo, you avoid some of from the phase issues.

Bad Mister!
Thank you for your deep explanation! This is exactly what I wanted to know!
I am playing my instrument through stereo system - Mackie console and studio montiors (HS8)..