Wanted to follow up with the conclusion of the story here to provide a cautionary tale to anyone troubleshooting an amp.
I finished my Crack amp today and gave the first listen with my old, shitty headphones just in case I had messed something up so I didn't blow my HD-650s. Fired everything up and got no sound out of the right ear. Turned the amp off, checked all the connections. Then I remembered that I'd used this set of headphones to test my non-functional SEX before sending it in.
I switched to an old pair of Sennheiser 598s I had laying around and everything's fine. Turns out that my Crack's right channel issue, and probably my SEX's right channel issue, were due to a blown set of headphones.
Two takeaways:
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[*]Always troubleshoot with a shitty set of headphones. I blew the right channel in the bad headphones when troubleshooting. Better those than my 650s.
[*]Get two sets of shitty headphones.
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