[Darryl Ramm]

Your troubleshooting points to what in the IO? It is a HD IO 16x16 analog?

Please just tell us exactly what you have done and what the results of the conclusive tests.

You want to simplify stuff as much as possible. I'd get rid of any mixer and connect up a set of powered monitors or simple analog headphone amp and use that to debug stuff. First thing is to reverse the L/R cables you at the HD IO outputs. What happens? Try all outputs, what happens? Start with a new empty Pro Tools session. Add an audio track using a know good MP3 or similar, and also try sine wave signal generator (on mono track feeding each channel equally).

Reversing any output pair in Pro Tools IO setup is also an easy thing to do and see if the problem swaps there. That tells you for sure if the Problem is Pro Tool software or hardware.

You have 16 outputs so check all of them. (try to get -10/+4 line-level settings the same). I could well imagine one faulty/damaged output port... and if so then you have the option of just using another port.

have you checked the DB-25 to TRS cables are wired correctly with a multimeter? Unlikely to be the wrong AES since they are TRS not XLR, but especially if you have three identical brand/construction cables I would double check they are indeed wired correctly (left side with a floating ground could explain everything). Of course if these identical same cables worked OK before then that's less important.

You can also use the signal generator plugin to play out each output and record than back into an input and make sure they all look the same in Pro Tools.

Pro Tools HD IO has no concept of "left" or "right" output, and you have multiple pairs of IO to try. if you are saying you a problem with the left channel on all the output pairs that does not sound like a HD IO hardware problem, it's Pro Tools or the downstream hardware.

Get a signal generator at a decent level at 1kHz plugin driving all all outputs and run along the back of the DB25 connector on the HD IO with a good quality multimeter. Does the AC voltage look about correct for the dbFS output level and the same for each output. Do this with probes at the HD IO DB25 connector not using any cables and check for bent or loose or corroded pins.

You often don't need to buy an ASC (support ticket) to have hardware repaired, but given the description so far here I would not be surprised if Avid support does want you to purchase an ASC so they can hand walk you though troubleshooting again. And that may well be you worth doing. If you are convinced the unit is faulty you can also just approach any Avid repair center or look for a used analog IO card yourself and try replacing it, that will likely fix and IO problem... but I'm not following from your description so far that you have really shown the IO is faulty.