Dave, the feedback issue is based upon whatever frequency vibrations are getting through the shelf into the TT. You might try using various weights on the shelf, and see if you can find a particular weight that brings it into a zone the TT suspension can handle. I think the reason that using the clamp makes it worse, is that the record is somewhat de-coupled when not using the clamp, so it doesn't get affected as much. From what you describe, the vibrations at a certain level and frequency are defeating the suspension and getting right up to the platter.
I think that there will always be somewhat more noise with a high-gain phono stage, but it shouldn't be a "hum". I don't think your Creek is the problem. But even a small grounding issue in the TT area will be fed through the high gain phono section and sound alot bigger than it might be otherwise.
Your room extension seems to be a "funnel" for reflected waves from the room, and focusing them on the equipment. This could be a large part of the problem. Try some standing diffusors or maybe even a bass trap in front of the TT/equipment. Even a couple of high back chairs in there to see if you can break up the wave-fronts, so you can see if there is any improvement.
Just a few guesses to try.
I think that there will always be somewhat more noise with a high-gain phono stage, but it shouldn't be a "hum". I don't think your Creek is the problem. But even a small grounding issue in the TT area will be fed through the high gain phono section and sound alot bigger than it might be otherwise.
Your room extension seems to be a "funnel" for reflected waves from the room, and focusing them on the equipment. This could be a large part of the problem. Try some standing diffusors or maybe even a bass trap in front of the TT/equipment. Even a couple of high back chairs in there to see if you can break up the wave-fronts, so you can see if there is any improvement.
Just a few guesses to try.