Wildly fluttering woofer

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.

Dear Heyraz, " I got the impression the original poster wasn't very hands on" is almost a compliment. I'm limited in knowledge and mobility due to a broken foot and a cast up to my knee and told no weight bearing. It was too difficult for me to reverse speaker cables which was my first thought. Somehow the tracking force was almost 2.6 gms which is strange since I'm the only person allowed to touch anything. I double checked the tracking force and set it to 2.0 gms. The arm is a Graham Phantom. Using a Yamaha C 60 preamp and correct tracking force there isn't any flutter. I'm hoping Art Ferris finds a problem with the preamp I sent him.
Thanks to everyone who offered their knowledge to "hands on". I'll post Art's comments when I hear from him. Katz1

Ouch! Wishing you a quick recovery. Enjoy the music while you're stuck inside.