hifiman5,
Thanks. My subs have never been spiked. Like my main speakers, the subs started out sitting directly on top of Symposium Super Plus platforms. While these platforms noticeably improved the bass of my main speakers in my old house, the platforms under the subs did not take care of the feedback oscillation.
The current setup that cured the oscillation was adding the Townshend Isolation Bars under the Symposium platforms. I'm going to try the subs directly on top of the Isolation Bars without the Symposium platforms for checking feedback and SQ.
My system, is in my new home in an unfamiliar room in terms of sound. It's like starting over. Producing the feedback and curing it was an education. Despite my heavy rack of maple and brass with Mapleshade Heavy Feet, my problem revealed that my rack is quite "active" and does not have a good defense against mechanical vibrations. I can take two approaches. One, is to put the Townshend Isolation Bars under my rack, or second, to do my components separately with Isolation Pods. I've also read here that Isoacoustics makes similar products with good results, but when I discover something that really works, I tend to stick with it.
Kenny