Some of them have capacitors installed backwards, some applications may trigger the problem, or they may fail with age:
To add a bit more to this story - we have a TM8 that was exhibiting the same problem. Low and uneven frequency response on Aux 3. I found this thread and took ours apart to find bulging blocking caps. Here's where things get interesting: I spoke to a friend of mine who does authorized service for QSC. Apparently in early production units of the TM's, the circuit boards were labelled backwards for the blocking caps, so although they appear to be installed correctly, they are backwards, meaning that if you send any of those outputs into a console with phantom power on, there's no...
