Is the weakening the same at all volume levels ? If so it's not likley the potentiometer. If the weakening is more pronounced at certain volume levels, specifically those you most regularly use (usually the lower portion of the volume range) then the potentiometer could be failing.
I have a feeling these amps use an ALPS RK27 motor driven pot, which can be found pretty easily online, and is pretty straightforward to replace. You'll just need to get the same impedance pot as the one already in there.
I have a feeling these amps use an ALPS RK27 motor driven pot, which can be found pretty easily online, and is pretty straightforward to replace. You'll just need to get the same impedance pot as the one already in there.