Okay... found a 100 ohm resistor in my parts bin, and used it to bypass #1 and #3, which gives clean audio again. Looks like we've found our culprits! I'll see if I can get one of our engineers to replace them.

Any idea why these would go bad, and if there's anything I can do to prevent it in the future? edit: could this be a result of the bad ground connection on the amp, that you mentioned in your earlier post?

Cheers,