Tube pre picking up feedback/vibration?

Your tubes appear to be microphonic - new low noise tubes are better than trying to quieting old ones with dampers. You said you had this retuned. Did you mean to say retubed? Also, re muddiness, are you by any chance using EH6922's?

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.

Your cable routing could be causing a problem. Do some 'process of elimination' experiments to see if you can pin it down. If you have a noisy tube, it will sound more like white noise/hiss, not so much hum. A hum in only one channel is often caused by an interconnect passing close to a transformer or power cord.

1st try just reversing the tubes from channel to channel. If the sound does not change with the switch it is not a tube problem. If it does, it is a tube. Replace it.

Next, look at all of your interconnect and power cords and try to keep them well away from each other. If you must have an IC near a PC, makes sure it crosses it, NOT running parallel to it. Also make sure that the IC's are well away from any transformers in any of your components.

If that doesn't do it, disconnect each source first the PC and then the IC's. If the sound goes away your problem is in your source or its connections. If is does go away, then reconnect them one channel at a time and see if it reappears.

If doing all of that doesn't get you a fix, you might consider that you have a cap going bad. Did ARC check out your unit or did you just buy and install new tubes? If the latter then perhaps you need to have a techie check this out.

Consider NOS 6h23's for replacement tubes for your unit. They are drop in replacements. They are very rugged and are fairly identical to Sovteks tone wise. Get them from a dealer who will test the tubes for noise (low) and microphonic's. I've used them for years in an ARC pre-amp. Interestingly the last time ARC had it they put some new Sovteks in my phone stage. Guess what, one of them failed shortly after I got it back. Shit happens - I doubt that ARC did anything wrong, and I imagine that they would give you a replacement tube if you asked (unless you got it too long ago).

Good that you solved your problem. Be thankful it was just a tube! :-)