Hello, I'm new to troubleshooting and repairing amps, but have a reasonable knowledge of electronics. I have an old Kustom KML 80 MosFet amp. The amp powers up fine, but the output is weak and distorted, even on the clean channel. Upon examining the power amp, I saw 2 resistors (the big rectangular ones, metal oxide i believe they were) were somewhat burned, and the circuit board under them was discolored from heat as well. I removed and tested them and they were off (684 ohm, theyy were supposed to be 750 +/- 5%) that didnt seem like a big deal, but I replaced them anyways, and tested 2 zener diodes and another resistor that were in line before them and they checked fine. I re-assembled the amp and still have the problem. 
Does anyone have any suggestions on where to begin, or am I going to have to test every single component on the circuitboard?