With the right tools you can VERY CAREFULLY test the coupling caps in the circuit. You un-solder the end of the cap that is going to the tone control and put the lead up in the air. Clip your meter to ground and the positive lead to the end of the cap sticking up in the air. Set the meter to DC volts. Then power the amp up. Let it get to normal operating condition. Put a signal about 200mV in the input and bring the volume up a bit and you can measure the leakage on the meter. A perfect cap should have 0V, but there is always some leakage since there is no cap that is perfect. Generally anything over maybe 0.03V is too much and throws off the bias of a following stage with a coupling cap. Since these are a tone cap, they are doing a slightly different function, but the idea is the same. You can also set your meter on capacitance checking, and check the caps, though be aware that if the caps have a lot of internal resistance or leakage it will throw off that reading and make it inaccurate. Also a DMM doesn't check the cap at the voltage that the cap works at, so that makes it somewhat inaccurate too. Also, you can't accurately check part values in circuit in many cases./
But it is simple enough and cheap enough you could just replace them also. Some Mallory 150's would work fine. I'd suspect all of the coupling caps and tone caps myself and test them all and replace any that let too much DC through them or read at a value far off what they are supposed to be. If you're really anal about it you could un-solder every part in the amp including resistors and check them and make sure they haven't drifted out of their tolerance. That's a pain and a lot of work though. I don't do it unless it is warranted.
Greg
