If it is buzzing from both channels and you are positive it is amp related, then the fact it occurs in both channels is indicative that it may be related to the power supply. That makes capacitors a good thing to check.
However, unless you pop the top and just happen to see a cap that is physically damaged, the key to repair is finding the bad part. Often that is impossible unless you have a service manual, appropriate test equipment (and know how to use it.)
There are also other problems that can cause an amp to buzz. If you don't know what you're doing, get it to a good repair shop. There are some substantial voltages in these things so you could shock yourself. An improper repair could also result in a fire hazard.
Also, sometimes one bad part can weaken or leave another part out of spec. As such, you'd want to be able to test the performance of the amp following repairs.
Figure at least $100 for someone to look and tell you what's wrong. The exact cost will depend on what they find, what the parts cost, and how difficult they are to replace.