ok, this is not good: looks like either all my sets of -ev/mil spec tubes are bad, or the pre does not like them. after powering down/disconnecting everything, switching stock 6n1p for the -ev, no issue. arcam seems intact, though now in win8 control panel/sound, when i test the 24/96 setting (default), it stutters on both channels...not sure what's up with that, but setting to 24/44 in sound devices fixes it (the test tones). the 24/96 setting has no issue during playback, but i don't have any high res files to test atm. i feel lucky i didn't blow up anything else. mark over at emerald physics told me an errant tube could have done much worse. even so, he's sending me a replacement dac 2. a total gentleman. thank you mark!