my experience with 80s/90s vintage electronics is mostly around failing small electrolytic caps. diagnosing and finding all the failed caps is very time consuming, hence the high repair bills. the parts themselves are cheap enough. also, some equipment is a lot easier to access the circuits for service (adcom, hafler) vs others (marantz, onkyo).
gads, back in the late 80s, I had a Marantz 510 (non-meter) fry and take out my old speakers. The amp was putting out like 120Khz at clipping levels, and smoke came out of the Ohm model H woofer and tweeters. Never let the magic smoke out of a good speaker before!
So I go to diagnose the amp, open it up, and holy crap, there's this heatsink tunnel thing with all the transistors on the inside, about 60 solder points on each PCBA to get them off the heatsink and swap any one of the transistors, and it was evident several were fried in addition to the driver board problem that caused the oscillation.
pic cribbed from http://www.classic-audio.com/marantz/0510m.html
I gave up and bought a Hafler XL280 and never looked back. Ohm Acoustics graciously traded my fried drivers in for new ones for a very nice price, the replacement woofers were better than the originals (carbon cones, longer travel), yet Ohm said they were optimized specifically to work with the same crossover and cabinet, just allow higher power levels which the smaller drivers were already capable of.
now, if you want a power amp thats -easy- to work on, open up a Hafler XL280 or similar....