Polytone mini-brute has poor sound.

[PfeifferElectronics]

This mini-brute came to me with a blown internal fuse (6amp output fuse). When I replaced the fuse, the amp works but has low output and very muffled and distorted. There is very little difference if I apply a signal to the effect loop in jack. So I'm thinking it is not the preamp... especially because the output fuse blew. So I took some readings. The power rails are at +36V and -36V at the output transistors (MJ4502/MJ802). DC on the output is fluctuating at around 30mV. I'm thinking it is further back in the power amp but not sure where to look next. Here is the schematic for the power amp.

[The Dude]

Have you checked the speaker with a known good amp or this amp with a known good speaker?