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I have tried every solution offered so far except testing with a multimeter.. I will do that ASAP and reply, thanks for the tip! I left everything completely unplugged from my I/O for a couple of days and then plugged it all back in. Bam! It worked just fine for about 30 seconds of playback. Then the 'distortion' came in.. light and intermittent. It got progressively worse to the point where my mixer was showing no signal coming from the left side at some points. This intermittent nature of the issue cannot be explained by a signal chain issue, also the nature of the distortion (digital-esque.. almost as if someone was wiggling the db25 connection to the I/O and it was shorting, and the part where I tend to lose huge bands of frequency i.e lows mostly drop but I have distorted highs) doesn't resemble any signal chain issue I've ever incountered. It genuinely sounds like a cable short of some kind but with all of the steps I've taken to troubleshoot, the only cause for that short must be the I/O.

Thank you so much for the helpful replies! I hope this additional description is useful and not 'word vomit' as it were!

P.S. monitoring the signal levels coming from the separate TRS cable for left/right output using the meter on my mixing board I believe would accomplish the same effect that we are looking to achieve with the multimeter, and indeed the signal from the one side is reduced and in some cases non existent.

*Edit* Yes, 16x16 analogue I/O and I have tested all the outputs

*Edit* I have followed prior to, and again after just to be sure, all of the troubleshooting steps mentioned in the previous posts and have experienced identical symptoms with no difference.. sorry if I am not providing enough info, but in each case the identical problem persists