Lets take a moment to recap (ha, pun intended) the situation:
Where did the polarity reverse info come from: In posts #3 and #5 above by Brian Ingwell, he confirms that there is an official QSC service bulletin regarding the possibility that the output caps may be installed in reverse polarity in some mixers. This info would not be widely available since QSC keeps both their schematics and their service bulletins secret. (side note, this is why I do not believe companies should be allowed to make service bulletins secret)
What do these caps do?: Even without a schematic, it is obvious that these capacitors are DC blocking capacitors. This is an AUDIO circuit, and as such, we do NOT want to have DC voltages flowing in or out of this device, because such a situation could either A) damage connected equipment, or B) damage the Touchmix itself via faulted or incorrectly configured connected equipment.
Where could these DC voltages come from?: They could either come from inside the Touchmix, or outside the Touchmix from connected equipment. I have double checked the Touchmix output circuitry with a precision multimeter. The Touchmix itself does not generate more than 11 MILIVOLTS of DC bias on the Main & Aux outputs. This is 0.011 of a volt. While not completely insignificant, it is negligible, and certainly does not justify the use of an electrolytic cap rated at 63v. Given that the installed capacitors are rated at 63 VOLTS on a circuit that has a mere 0.01 volts of DC on it, the only logical reason to use capacitors rated at such a high voltage would be to protect against +48v phantom power.
How / Why would +48v Phantom power get applied to a Touchmix OUTPUT?: A "small" mixer like the TM8 or TM16 may be used by drummers, keyboardists, or anyone else with a bunch of instruments that they want to do a personal mix on, before sending that personal mix to Front of House (FoH) using the Touchmix's Mains and / or Auxes (which are then connected to a FoH mixer INPUT). When a small mixer is used as a "sub-mixer", there is ALWAYS a danger that the FoH mixer has phantom power turned on. On many large FoH mixers, phantom power is an all or nothing proposition. All channels on or all channels off. Other FoH mixers control phantom power in blocks of 4 channels (or more). It is trivially EASY for +48v to be accidentally applied to the Touchmix in a situation like this. Phantom power is applied to microphones (some direct boxes need it too) via pin 2 of the XLR jack / plug. In order to properly protect a Touchmix (or any other sub-mixer) from an accidental zap of +48v, we would need the POSITIVE side of a capacitor rated at least 10% higher (preferably much more) than 48v connected to pin 2 of the XLR connectors on all the OUTPUTS of the sub-mixer.
Edit: (George - oops, I see you posted again while I was posting this - good luck in your quests)
I hope this makes sense, and also helps any future readers of this thread. I really don't know what else to say regarding this subject. I've done my best to explain in simple terms why I think the QSC engineers built the product this way, and why an assembly error where these caps are installed backwards is a grave fault that should be publicly acknowledged.
My final thought on this subject is: If you (the generic you, to anyone reading this in the future) ONLY use your Touchmix as a MAIN mixer in situations that you control closely, and you have complete confidence in all the equipment that you attach to the OUTPUTS of your Touchmix, then you can install these capacitors in any orientation you want. The DC bias of the Touchmix output circuit is approximately 0.01 volts, and it is generally accepted in the electronics industry that electrolytic capacitors can survive for extended periods of time, possibly their entire lifespan, with up to 0.5 volts of reverse voltage applied. So install them how you want... just be absolutely sure that +48v phantom never makes it to your Touchmix if you decide to go with the capacitor negative to XLR pin 2, which is how some boards have been incorrectly assembled by QSC.
