Tonearm microphonics

I'm glad this subject was broached.  I've been in contact, just this week, with two well known experts in the hi-fi field, concerning acoustical feedback in my TT, when playing low frequency LP's at a louder than normal level.  The bass begins to build on itself and in a few seconds, I have a low frequency feedback problem.  In reading this thread, I noticed it was mentioned, one should never get any sound, when tapping on the TT plinth.  I have a VPI Scoutmaster, with a JMW-9 signature tonearm.  The cart is a Dynavector DRT XV-1s.  With the tonearm at the end of the record, I can tap on the plinth with my finger and clearly hear the thumping noise through my speakers.  I have tried a couple of methods to dampen the tonearm (haven't tried the shell yet-there really isn't one) to no avail.  I'm thinking this has to be contributing to my problem.  I have also tried dampening the plinth by placing sealed bags of sand on top of it any trying the same test.  The tapping noise is still there and has not been diminished in any way.  I wanted to employ a subsonic filter that would eliminate all frequencies below 10-15 hz on my subwoofers, as when they're turned off, the problem goes away, but can't find one.  I'd rather not go the KAB way as I think I would be losing some of my good frequency response.  I realize most of my problem, must be coming from my tonearm, but was curious about the plinth comment.  Does anyone have words of wisdom for me about my tonearm and plinth?  BTW, I've tried rearranging my subs and have significant room treatment.  My room is not just an untreated shell.  

Karl:
When trying the tapping test, be sure your stylus is on the album at the end of it, where it tracks out.  If at rest or cued up, you shouldn't hear anything.  I use all balanced interconnects, except from my turntables.  I have to use SE there

BTW, I have one TT on an inner tube and spiked, the VPI is spiked (it comes with it's own adjustable spiked feet) onto the discs that come with spikes.  The spike discs are on rubber compressor feet and my TT platform is isolated from the wall.  I don't think it's an isolation problem.  I can tap on my platform, when the record is tracking out and I hear nothing through my speakers-standing at my TT position.  Thanks for the input-always wanting to learn.

Hi Raul:
I've seen your name mentioned in several discussions, but don't know anything about you and really don't need to.  From other's comments, people look to you for advice.  I'm 63 years young and have been into sound since I was four.  I was given a "real" record player for Christmas and after seeing my uncle string speakers across my grandparents room, went home and disassembled my record player, to see if I could do the same.  My dad bought me a 4" speaker to play with and I've been hooked ever since.  I'm going to address some of your questions, but I need to give you info about my equipment and room.  It's 19' x 21' x 10' high.  I have a dropped acoustic tile ceiling that's about twelve inches lower, with 8" fiberglass batts resting on that.  I have about 12 acoustic panels made of 1" hard fiberglass, covered with speaker cloth, that are on the 45 degree walls leading up to the ceiling (jump ahead to get a description of the walls) and are 2" out from the wall itself. I have 4 home made ASC-like tube traps strategically placed.  I think my room is too small, for the energy I'm trying to introduce in it.  I'm using 2 Fathom 113 subs that are diagonally placed, almost facing each other, about 4-5' out from each corner, with one being set 180 degrees out of phase, to help with cancellation.  My room has knee walls aprox five feet tall, then goes into a 45 degree angle to the ceiling. The ceiling is about 8' wide. I read Jim Smith's book and decided to place my TT on the left wall, between my listening area and the speakers.  The speakers are about 8.5' apart, 3 and 4' from the side and rear walls and 9.5 feet from the listening area.  Because my floor is on the second story and is on a suspended floor,  I eventually had to separate the TT platform from the wall and using heavy duty springs, eye bolts, steel cables and turnbuckles, successfully achieved a kind of floating shelf.  No more problems with walking across the floor etc.  To your question about different carts.  I have a 1974 Pioneer PL 71 that has a Koetsu Urishi Black on it and I can run it a good 3-6 Db higher, before I get any feedback-maybe even higher than that.  I have a Sony ESD 2000 preamp I run my subs through, for convenient remote control and separate gain control.  This way, I can vary the volume of the sub, depending on what record I'm listening to.  Today I added some dampening fluid to the VPI, but it didn't seem to help. I once had a Lyra Delos on the VPI, but have added another sub since then.  I'm thinking I had the same problem then. I'm not saying I can't listen at a comfortable level, but if I turn it up, (85-90 db) I get an acoustical feedback problem.  I was told by some of the best, that's just the way it is.  I would appreciate any solace you can provide.  BTW-no woofer movement, when not playing.  Wish I could find a subsonic filter, single or double channel that would filter out 15hz and down.

The do level of the subs are around 45-50

Hi Raul:
I have a couple of Wilson Maxx II's.  They're rated @ 20hz, but they don't quite get there.   If I place my subs in front of the speakers, won't that block my main bass?  I have had the subs facing each other before, but they were behind my speakers.  That placed them too close to the corner and overemphasized the bass.  I saw a picture of a guys room and he had his on each side of his listening chair pointed away from him.  I need to build a couple of diaphragmatic absorbers, as well as a couple QRD-13 dispersion panels.  Won't help my current problem, but may help the over all sound.  Do you incorporate these devices?
I have my sub sourced through my tape out on my Calypso preamp and have the subs rolled off around 35hz, so most of the main bass will go through my main speakers.  I kinda like to play with the sub volume, specially on old LP's where they rolled off the bass to high.  I know it's not "real", but still sounds better to me, than the rolled off sound.  When playing quality LP's and the occasional CD, I set the sub volume and leave it.  I was listening to The Thompson Twins "Through The Gap" the other day with my mains turned down low.  I heard some really good low frequency, I hadn't realized was just coming through the subs, so I think I get them matched up pretty well.  It's fun to just listen to the subs sometimes, just to hear things like that.
Sounds like your subs are really good.  Wish I had shopped around more, but mine are OK.  I'm still open to suggestions.  As I said, wish I could find a filter to block everything from around 15hz and down.

Raul:
True story about me supplementing my bass.  The Wilsons put out such good clean sound, I hate to roll them off, besides, I'd have to get an active crossover to roll the lows off @ 80hz.  Maybe I will try it.  If I installed a filter, that only cut 15hz and below, that wouldn't degrade my sound.  I realize we feel some bass we can't hear, but 16hz is low enough for me.  I have a Bag End sub that they say goes down to 8hz and the walls shake, as well as other things, before you ever "hear" any sound.  I had it in a Home Theater system, but it's just sitting in the spare bedroom right now.  I will continue to experiment with sub placement and frequency adjustments.  Thanks for taking the time to respond.  If you have any other suggestions, don't hesitate to tell me.