Adcom GFA5500 poor bass response. Can anyone help?

Genuinely poor bass response (a fall in output vs frequency) can only realistically be caused by a failing (electrolytic) capacitor in the signal path that has reduced in value and/or developed a high E.S.R.

A scope check will prove if there is a problem.
 
C3 is in the feedback path and that can cause reduced bass if bad.

Craig
Replacing C3 just worked perfectly for me yesterday. Getting to it wasn't too bad. I used a closely sized allen wrench in the transistors mounting hole to GENTLY rock and break the transistor from the thermal tape. The only problem I had was one nylon screw holding two transistors to the heat sink lost its head. There was enough of the screws shank to fit a 6-32 nut as if it was threaded rod. I needed to file down the nuts perimeter for the flats to clear the transistors body. I just got a dozen new ones on Ebay. 5500 sounds really good now.