Please Help Ambient Bass Resonance

I have a problem I just noticed last night. I believe it's mechanical. Perhaps the vinyl folks can explain what I am hearing and suggest a solution.

First my equipment: Lyra Delos Cartridge; VPI Classic TT; ARC PH-8 Phono Pre; ARC Ref 5 SE linestage; and ARC CD-8 CDP.

The Delos output is .6mV. The PH-8 fixed gain is 58db and its S/N spec is 75db. The Classic is grounded to the PH-8. I also use a Delrin clamp and periheral ring.

Because the Delos output is about as low as ARC recommends for the PH-8, I have to turn the Ref 5's gain up to about 70-75%. Normally, noise and hiss is acceptable.

Last night I was playing a record and I turned the gain up to 75+% and I heard a low freq bass ringing that started to feedback on itself and got louder as I left the gain up. It would stop when I turned the gain down to 65 or 70%.

I believe its mechanical because I did some experimenting with the rest of my gear. Step 1: I clicked the Ref 5 to the CDP and turned the gain up to over 95%. Dead silence. Step 2: I turned off the TT and put the stylus on the record. I heard the bass ringing. Step 3: I disconnected the lemco connector from the junction box -- that is I disconnected the cartridge from the PH-8. Turned the gain up. Dead silience. Step 4: I reconnected the cartridge and kept it suspended off the record, motor on or off, turned up the gain. Silence.

I believe that what's happenening is that the record is acting as a low freq. sound collector and the vibrations are being picked up by the cartridge and fed back on itself. Hard to be sure about the Hz level but it may be in the 60 Hz neighborhood. I surmise this because I get the feedback with or without my sub, which cuts off at 50 Hz and the him seems to come out of the bass woofers.

As I said, the problem becomes apparent when I turn the gain up past 75 or 80% with the TT on or off, when the stylus is on the record.

Any explanations and suggestions? Did I find Fishing's missing bass? Check his OP. LOL ;>')

Seriously ... what is going on?

P.S. Just tried something else. I checked the hum situation without the peripheral ring. Still have bass ringing feedback, but I had to turn the gain up even higher for it to kick in. Again, it only happens when the stylus is on the record -- regardless if the motor is on or off.

Thanks guys