As noted above, the likely problem is part of the actual virtual sound, and not midi. The midi CC - if you try to use it, will have no effect. The built-in sound is - I guess - pretty much fixed, and you have little control over it. Maybe there is a way to completely replace the sound system, so that a different soundfont is used?
I have noticed a similar thing when I'm using a soundfont on my PC. The particular one I use, which I quite like otherwise, does seem to have a fair bit of echo/reverb built in, and it's not subject to change via normal CC. Luckily not 'too much'.
Either the reverb was already part of the sound when it was originally sampled to create the foundfont (or whatever type of file it is) or it has been added at a very early stage of processing. You can ADD such effects easily enough, I'm not sure it's possible to take them away unless you can remove or reduce the process that ADDs in the first place.
Geoff