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
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?
Remove the top of your preamp and with the system on tap lightly on the tubes. The microphonic tube will be located by using this method.
Thanks . Will try to get this done today. Yes, retubed. Thank you spell checker!!
Tubes were purchased for ARC, 2 weeks ago, and I assumed they would check for defects before they were sent out. Go figure!
Again, thanks for the advice.