I'll take a second shot at this one -
I'm not sure but it appears that Brrgrr's complaint is 'muddiness' and when he taps the case he gets feedback from his speakers caused, we must assume by microphonics. He connects the microphonics and the muddiness, or at least, he suggests a possible connection.
He has just replaced the tubes. Last I heard ARC was using EH6922's in its stuff and as replacement tubes. To be blunt, I have used these tubes in an ARC pre-amp as well as a couple of tubed CDP's and amps. I found them to be muddy in the midrange. I also found them noisy (very low level though). I never noticed a problem with microphonics, but then I never tap tubes with anything to find out. If I can't hear it it must not exist. :-)
I think it is entirely possible that the OP's only problem is tube selection and his problem could be solved by getting different tubes, and not just new replacement ones from ARC. If this amp really does stress its tubes, as Marakanetz suggests, he could try some old stock 6h23EB's. These tubes are not warm. If the OP wants some warmth he might try different tubes in the second stage such as the new production JJ's. Worked quite well in my ARC preamp.
FWIW.
FWIW.