You could try opening the amp up and spraying http://www.amazon.com/CAIG-DeOxit-Cleaning-Solution-Spray/dp/B0002BBV4G on all of the pots and switches as it is likely a build up of oxide if one channel is low. This may be all that a service center might do and you can do it yourself very easily Theres loads of info on the net on how to do it
If this doesn't work and the noise is still there, then it may be a faulty capacitor. This is where you do need some skill in finding it and replacing it... If this is the case then I'd probably look for another amp if you are not confident in taking this route. $150 can get you some pretty good vintage stuff if you keep looking on craiglist and fleabay
Words of wisdom. Also try aggressively working ALL of the pots and switches while the amp is off. Cycle the switches on and off rapidly 20 times each, and rotate the volume control swiftly through its entire range 20 times. If you turn the amp on and it works well right away, then the issue for sure is that the pots or switches need cleaning with deoxit. That at least is an inexpensive fix.
Otherwise, and please don't take offense, but that particular Denon just isn't worth spending a lot of money to fix. It's not that it's a bad unit, I'm sure it can sound quite good, but it's not anything worth investing more money in at this stage of its life. You are better off buying something else, be it a nice vintage piece, or a modern one.