i recently bought a pre-owned A-800 Pro because i wanted to try this keyboard… so, not bad at all, but the aftertouch was a nightmare!
I did some research and stumbled upon this posting in the Cakewalk forums:
Since my device already lost warranty some time ago, i just dared to try this modification/fix which was quite easy to do. And it actually helped a lot! Now the aftertouch is quite useable - not to compare with my NI Komplete Kontrol, but much better than it was before.
Another guy soldered some additional resistors to the controller chip (on an Edirol keyboard, which seemed to be the predecissor of the A-Pro keyboards), which might have improved the aftertouch action even more… but i thought soldering was a “too heavy weapon” for the first try.
There is a picture in the posting at the end of the thread which actually looks quite similar as the circuit board of the A-800, so this might help you identify the little yellow pot which adjusts the aftertouch sensitivity…
And be aware that opening your device causes the loss of warranty - and everything you do, you’ll do at your own risk! Don’t blame me if you damage your keyboard!