I went back and read the original thread that got you to this point. So to restate the distortion description is that it is on the attack transient and does not follow the note decay, correct?
Have you gone through the amp and tested the dc voltages in the power amp and compared them to the schematic? Do you feel safe in reading voltages while the amp is plugged in and turned on?
Depending upon the exact model that you have, the overload protection circuit may be the cause of your problem. Very early A's did not have this circuit, so you will need to check exactly which model power amp you have.