Contact cleaner
Can be bought through the Radio Shack catalog, etc. I would suggest cleaning that monster of a switch, the pots., etc. alongside the mechanical and continuity checks mentioned above.
It is possible that there has been a component failure in one of the amplification stages, so if the above doesn't fix it, try swapping leads at the heads. If the problem switches to the 'left' channel, then it is head alignment or some closely associated mechanical problem.
If the hiss remains, before you start checking the amps, check the leads - they are awfully thin and, if damaged during installation, break over time.
If the above fails, try injecting signal at each stage, working back from the final stage. You will probably have three stages of active electronics involved: power amp, 'pre-amp' (volume control stage) and the first amp and equalization right after the heads. Again, anytime you can swap channels to check things out, do - faster and easier.
A wild card, but magnetic fields? Demagnetization?
