Thx for the response ... I'll get to the rest of the reply later as I'm still troubleshooting ... but I wanted to address a quickie:
You should have been testing only for DC offset, from the start. It is not a sufficient test, but you would have seen the problem without risking your expensive IEMs or your junk cans. Nothing you've reported about how the problem sounds tells us anything more than a high DC offset measurement would have.
Maybe you're annoyed now, thinking I'm armchair quarterbacking. But, a DC offset test is one of the standard testing steps. When you rolled that op-amp, you should have gone back through that test sequence. Plugging cans in is four paragraphs down the sequence, and it recommends starting with the cheapies.
In my orig. post, I noted the gazillion times I've cycled thru battery changes and OPA rolls and misc. tweaks. Even batt changes requires I remove the cover, creating some risk for messing something up. My routine was to use my default cans, the Senn IE-8s. Frankly, then, even with $$$ cans, I'll still take the risk since: the Pimeta2 has proven to be v. reliable/durable; the sequence method is mostly (IMO) for initial builds/testing; TIME=$$ (especially in today's economy) and, my 'problems' generates content for HF and more biz and troubleshooting-skill-building for you/readers-of-this-post ;) [For more enlightened philosophical discussion on these issues, see my HF Profile. :) :) ]
This just in ...
I just discovered that the orig.-installed Q3R and Q2 are not 2N3904 but 2N5088. Although I trashed the Q3L I removed earlier, i now assume it was 2N5088. I got mixed up because on your schema, you label the defaults as 2N3904 but on your Parts List they are Alternatives.
To be cont ...