Tube pre picking up feedback/vibration?


Recently got a pair of Magnepan 3.7's and a Sanders Magtech.
Have an old ARC LS15, newly retuned.
Have noticed a muddiness in the bass, kind of a one tone bass at times. When I tap on the preamp cabinet, get a distinct sound transmitting to the speakers, so
I gather I'm picking up some feedback. The pre is rather cole to the speakers, and it doesn't have the isolation of more current models.
Can you guys help with suggestions to stop this?
I'm considering a new preamp, but never noticed this with my previous speakers.
Are there isolation devices to deal with this, tub dampers, etc???
Changes the wires from the Pre to the amp, Putting in my Verbatim wires in place of the Nordost, and that helped some. but want to remove this source of pollution from the system, obviously.
Thanks
Carl
brrgrr
Finally dug up an old set of Sovtek tubes, probably from many years ago, that were in a box in the basement. Put them in and took out the new ones ARC had sent me ...and no more noise, microphones, whatsoever!
To say I'm disappointed that they would sell crap like this is an understatement. These tubes are marked by them for the specific spot on the circuit board, but I guess no one actually tests them for micro phonics. Do they need to actually be installed in a preamp for thai to show up?? I figured one of the reasons that I should buy from the source, so to speak, was that they would send me something that wasn't a crap shoot.
Makes me wonder if I should look at this brand when I upgrade.
Anyway, problem solved. Now to get a replacement set that only make music when the signal says to!!..))
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! :-)
John, I'd never suggest changing value of NFB resistor. All I advised is to check if this value had ever been changed by previous owner or any prior