How good is the Obsidian plinth of Technics SP10 ?

I have an SP10 MK II with the Obsidian plinth. I am using a Basis Vector arm with a Soundsmith Zyphyr MIMC. My audio room is on the second floor so I have always mounted my turntables on the large Target  TTW1 wall mount rack. I replaced the cheap target shelf with a 3" maple platform.
I removed the crap feet that came with the obsidian plinth and have the original version of the stillpoints pointed up to the plinth. I had my SP10 rebuilt by Hans Lazuadi as well as he fabricated a brass mat that was screwed to and balanced with with SP10 platter. My system also has two Seaton Submersive subs that I use in both stereo and surround modes.
Using the subs in stereo mode I can drive the volume up pretty darn loud before I get any sort feedback issue from the turntable.
   I too have wondered about other plinths but have been hesitant to invest big $ in a custom plinth not knowing if the incremental improvement would be significant.
  I will also comment that recently I have been ripping my vinyl to DSD128 using the Korg DS-DAC-10R. I find the playback of ripped vinyl via my Lumin A1 to be at least equal to playing the vinyl and superior in the aspect that I can truly crank the volume with two subs well beyond reason without any feedback.

Hi Tellefsen

 Something is not quite right if you are getting feedback from your turntable at any volume.
  

I guess here is where I pipe in to say that I commissioned the construction of a slate slab, 19x23x3 inches, to fit the Mk3 chassis.  I had a hole cut right through the slate large enough to allow me to position just about any tonearm.  Then I had the same company make me some slate tonearm mounting boards.  The slate weighs about 70-80 lbs, so that was a good plinth. But after a while I added a cherry wood base, about 4 inches thick. Into the wooden base I incorporated the dampening mechanism that Albert Porter uses, a block of solid brass (Albert uses solid iron), into which I insert a threaded brass rod, about 5/8" in diameter.  The brass block is firmly bolted to the wood and sits directly under the bearing. The brass rod contacts the underside of the Mk3 bearing well.  That whole thing is bolted to the underside of the slate.  Total weighs maybe 100 lbs. This sits on Stillpoints.

To Downunder:  The two subs I am using in stereo mode are the Seaton Submersives. They are dual 15" subs with a 3400 watt amp each. They are flat to about 10-14 hz...So they can definitely excite the cartridge/tonearm/table system into feedback with a record with a lot
of low frequency info at high volume (think Donald Fagen - Morph the Cat)...It never happens with just my main speaker running full range (Green Mountain Audio Continuum 3) which have a -3db at 27Hz.

@lewm

feets Audiotechnica AT 636 does not exist if I search on the internet.

Are you sure of the acronym or maybe are you confused with the AT 615?