well, another "false alarm". turns out the sovtek tube i had in the left mono in the direct-coupled stage was hummy. subbed out each mono's Sov at this position with an OS RCA coin-base 6SN7GTB and all is good :)
learned an important lesson - ALWAYS have spare tubes, preferably of a different brand. can't wait to try different 6L6's. anyone want to happen to try my Sovtek 6L6WXT+'s in exchange for some other brand? be warned, i'm kicking the pants out of them in this amp: 500VB+, 55mA per tube (27.5W dissipation). way above the GC's screen rating but they're doing OK and most decent tubes should live.
tweaks like NFB can wait for vacation - something I want to fine-tune, probably. thanks everyone!