If the receiver is shutting off at high volume with heavy bass I don't think it's the receiver's fault. Receivers have built in circuitry to protect the components of the receiver as well as your speakers from damage. If the electrical current demands you are placing on the receiver are beyond it's operational capacity it will shut down in order to prevent damage to the receiver. Basically, it's probably not in need of repair, but if you continue to run the receiver at high volume with heavy bass it may need to be repaired eventually.
Receivers that have problems with shutting off normally do so for one of two reasons, a short circuit or an open circuit. If it was short circuiting it would shut down at very low volumes, an open circuit would probably trip the protection as soon as the relays close (the second click after you turn the receiver on).
The AVR 20 isn't a very powerful receiver, and appears to be rated 65 watts per channel RMS. It would certainly be enough power for all but the most demanding speaker and listener in a medium size room. If your speakers are very inefficient, or have a low impedance and you are trying to fill a large room with concert levels of output I would expect that receiver to struggle and shut down. As I mentioned before, if you keep pushing the limits of this receiver you will total it and it will be worthless.
What kind of speakers are you using with the receiver? Another thing you might want to do just as preventitive maintanance would be to check your speaker cabling and make sure there are no short circuits.
