i'm sure the forum heavyweights will chime in soon (they helped me work through a 2440 that had the same "one channel out with loud hum"), but to get you started:
Deoxit the controls
inspect the boards for any bulging caps or burned resistors
try running through the preamp of the 2215 to another amp (use the tape monitor) to see if the problem is in the preamp or the amp
if it's the amp board, and you are so equipped, test the transistors.
In my case, I (1) used Deoxit on controls, (2) verified it was the amp side, (3) recapped the whole unit, and then found a burned resistor which pointed to shorted transistor somewhere. Pulled a few and they gave no reading on my multimeter, so pulled them all and replaced with NTE parts.
I am also working on a 2215 (luckily both channels work) so I know that for a complete recap, it's about $30 in caps for Panasonic FC's from digikey. The amp board is laid out just like my 2440, so a complete replacement of the transistors would be about $40 for NTE parts. (More if you use new old stock transistors...but on a 15w/ch unit like the 2215, i have a hard time getting spendy on parts).
Plenty of great how-to's around here to walk you through re-capping, replacing transistors, etc. I never would have tackled it without AK.