Hi Parker,
Welcome to Yamaha Synth!

In actual fact, the Output level of the keys across the MX should be equal in Output, what you are dealing with, in all likelihood, is the response of your sound system -(yet unmentioned). Make sure your high frequency driver(s) are working.

I just played an MX88 in Indiana last week, they had it through a typical keyboard amp. I had to show the sales associate that there was nothing at all coming through the amp's piezo speaker (famous for blowing out - leaving you with nothing above 4K).
Don't look to the keyboard to compensate for something that might be the speaker's fault. It may not be able to overcome a speaker that can't reproduce the frequency.

In the situation in this particular store, if I EQ'd the Piano or even if I EQ'd the amp, the poor keyboard amp had nothing to reproduce signals above 4kHz, so that was a waste of time. 4kHz is very high, to be sure, the highest Key on a piano is 4,186Hz... but music gains clarity in this critical region. All your upper harmonics live between there and 20kHz. Heck, the reason music sounds terrible over the telephone is it can't do anything above 3k (or under 300Hz). Highs give clarity, sharpness, focus.

Since I had traveled all the way from New York, and since they had yet to actually hear what the MX88 really sounded like... We switched it to a full range sound system - plugged it in in stereo. The folks in the store couldn't believe it was the same keyboard. I mention that just to impress upon you the importance of the sound system you plug into... the weakest link in the chain will cause the most distress.

You may or may not be able to compensate for it by editing the sound. Sending more highs to a blown component will not necessarily fix the problem. Just like the Rotary Speaker or the Auto Pan cannot be realized in mono, you can (incorrectly) blame the Keyboard, but the better solution is a full range, stereo sound system. Both of those effects require two speakers and two ears to create the illusion of movement... same with all of the stereo effects in the MX88. (Instead of the expected "movement" you get a kind of warbling in place - so totally different from what is built-in to the Keyboard.

I suggest, either mention the type and nature of your sound system, whether or not you are connected in stereo, and whether or not you are sitting in front of or behind the speakers when you evaluate the sound (don't laugh, I work with many world-class keyboard players who have their speakers out toward the audience facing away from them, yet they wonder why the sound is dark! Go figure)... we need that additional info, particularly to offer help here. I'm not saying this is what you're doing, what I'm saying is, I cannot really know the cause with just the info you've provided thus far. (I do know the MX88 is not at fault)... we need the info to offer meaningful solutions.

The keyboard MX88 was carefully programmed to be balanced across its range, you can adjust the instrument's EQ, but it is going to be to little or no avail, if your sound system is at fault and cannot reproduce certain frequencies. Bass amps, obviously cannot do pianos...

Let us know.

In the meantime...
Get a set of quality headphones, connect to the Phones jack. You'll see (I mean, you'll immediately HEAR it), prove to yourself that the Yamaha engineers did a really good job of creating an instrument that is well balanced across all keys... then you'll agree with me: the solution might well be you need to fix your speaker system!

The MX88 features a high quality 5-band MASTER EQ, which can be used specifically for shaping the overall sound of the instrument in the sound system and room you are in. (Each Voice can also be EQ'd, with its own separate EQ, to make it work within a mix of onboard sounds) But this Master EQ is overall, it will affect all MX sounds. We need to determine *where* exactly, your particular issue is in order to best provide a solution... is it overall? as in every sound seems to lack high frequencies, or just particular sounds? ... this way you can select to EQ in the most appropriate place.

Thanks for your in depth report. I am actually very aware of most of what you address having been professional for 40 years. My system is fine. I can EQ the piano like I want it to be - where it is even in boom volume from bottom to top. But then the EQ is so exaggerated all the other performances completely don't work.
My Kurzweil PC3X and every other K has something that each voice can be set by a simple -64 or +64 that shifts the weights of the volume from bottom to top that is exactly what I'm talking about.
This pertains to just the piano sounds. And its only the first couple which are the only realistic ones.
Anyway I got the MX88 because if has a lot of good things about it for a gigging keyman, esp. that it is light in weight.
I'm never taking that 100lb Kurz out again now that I got the MX88.
I'll have to come up with someway to pop a button to change EQ.
thx.

Thanks for your in depth report. I am actually very aware of most of what you address having been professional for 40 years. My system is fine. I can EQ the piano like I want it to be - where it is even in boom volume from bottom to top. But then the EQ is so exaggerated all the other performances completely don't work.

Sorry, I guess I wasn't clear. You can STORE a different MASTER EQ setting offset for each MX PERFORMANCE, so editing one should not affect any other PERFORMANCE.

Call up the PERFORMANCE 001-128
Press [EDIT]
Select "Common" > Enter
Select "Master EQ" > Enter
This will EQ the Performance... all 16 Parts of the Performance will be affected by this EQ.

If you want to EQ just the Piano Part alone within a Performance, assign an Insert Effect, "VCM EQ 501"... this will allow you to EQ just the Piano Part.