Wildly fluttering woofer

Dear Heyraz, After all the repositioning everything was fine for the two weeks after I installed Herbies devices. I'm waiting to hear how the preamp checks out. Thank you for taking the time. Katz1

While you're waiting for your preamp to come back, you might find some of these tools at the Vinyl Engine website interesting.

http://www.vinylengine.com/cartridge_database_tools.php

Heyraz, the argument against feedback is that when it was happening, it was only in one channel. If it was mechanical feedback it would have been in both channels. The two phono channels of the preamp are right next to each other.

Cartridge and arm mismatch will cause that problem. I would blow woofers all the time. You have to find out what the arm is resonating at. Get a good test record. I use a Shure audio obstacle course ERA 4 to check mine.

Atmasphere,
valid point, but I didn't know if the offending speaker was right next to the turntable or not. That's a great way to create feedback. I also didn't know if the speakers were in phase. If it happened to me I would have reversed channels via the interconnects between each component till I isolated the problem. Or try another tube as you suggested, but I got the impression the original poster wasn't very hands on.
As far as a noisy tube goes, I don't know how his preamp is designed (tubes carrying both channels or one). The problem seemed subsonic and only in one channel, which rules out the power supply (unless it's completely dual mono). There was no mention of how much volume, if any was required to cause the problem.
The problem actually sounded more like motorboating in one channel to me, which points to an amplifier's feedback loop, and a bigger problem than a noisy tube.
There just wasn't enough information to formulate a reasonable cause as there was no mention of a process of elimination.

I'm interested in the outcome.