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
Marakanetz, I find your BMW 5 series analogy especially useful since that has been my car of choice for more than 10 years. Come to think of it I have never seen a 3 series with the lack of options you mentioned either.

BTW Audio Research volume pots can be easily adjusted by simply changing a single resistor.
That single resistor is a feedback resistor. Altering this resistor from default setting to some magnitude may bring the amp onto oscillation or out of stability and another level of microphony(good point to check btw). The best what can happen is sacrificed dynamic range and transparency. I believe and know that tubes are not microphonic and amps/preamps are. There are plenty of good designed tube preamps with no microphony and my eyes and hands had seen them. Did anyone have an experience with AES by Cary little tube preamp?
As to BMW 5 or 3 series, in other than US countries people are still paying top dollar for these automobilles, but have still options not to have all extra items such as auto-trans, rear climate, auto-seat, power windows etc...
The resistor change is Audio Research's recommended fix for adjusting volume control setting. Ideally the volume control should be in the 11-1 o'clock position at your desired listening level.
Still not happy with the results of the new tubes. Emailed ARC and they basically said all is well. Now I notice a low hum from one channel and reversing the wires leads me to suspect the tubed. What exactly will a bad tube sound like? Hear the hum with volume all the way down and the cd player stopped. Like I said I reversed the wires to the amp an
d the problem followed the change so I have ruled out the amp. Ideas?
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.