If you don't have a firm grasp of what you're doing - and no offense, but it seems that you don't - I implore you NOT to poke around inside the amp, whether it's powered up or not.
There are hundreds of volts, both AC and DC present all over the place, and one false move or slip with a meter probe (or anything else conductive) could literally be deadly.
Even with the power switch off, the power supply capacitors can keep a charge of several hundred volts. Again, all it takes is one wrong move.
I'd be happy to take a stab at it, for the cost of parts and shipping both ways, with no guarantee other than I won't give it back to you in worse condition than I got it. Building and fixing has been my main hobby for over 20 years.