How to eliminate TT feedback/vibration

1st Thing I will tell you - I have an OLD TT. It's a Pioneer PL-71. ( I'm looking at a Clearaudio CMB ) My TT is placed out in front and to the left of my left speaker(per Jim Smith's book.) I made a platform of MDF attached to the wall where my TT sits. At no point does the platform contact the floor. When I play certain albums-several in fact, at mid to high db levels, I get either an acoustical feedback or vibration feedback from low hz. I have tried to isolate my TT using something like sorbuthane balls and foam. I have not tried any of the commercial cone devices advertised. (Do they really work and if so-what's the best product?-no salesmen please). I don't know if my problem lies with my TT, or my platform it's sitting on. I have a friend who actually hung his TT from the ceiling years ago, and I may have to do the same. Thought I would inquire if anyone knew the answer before I started experimenting. Will a higher end TT like the Clearaudio cure my problem? Is it the platform I made-or do I simply need some isolation cones?