I recently took a power amplifier that a friend recommnended, typically it was for PA purposes but with very good specifications, 250Watts per channel into 4 Ohms. I have been hacking away at it as I have a little knowledge in electronics.
I replaced all the electronlytics capacitors with Panasonic, Vishay and also changed some of the ceramic capacitors to high stability types, I changed and opamp in the input buffer to an LM4562 from a 5532. I am on the cusp of trying it out on my PMC monitors. I just need to replace a burnt out varistor on the mains board.(I will test it on some cheaper speakers first just in case there are any issues) the amplifier sounded as good as a NAD power amplifier I had been using previously which is a good sign. So now with the changes I am looking forwards to hearing it.
It can be very rewarding if you are prepared to learn and use a soldering iron.
cheers