Lewm nailed it. Anything you can do to rigidify the floor under your turntable will be a good thing - like concrete blocks wedged between crawl space floor and floor joists.
Can a turntable ever have enough isolation?
I have a Pioneer PLX-1000 turntable mounted with minnesota pc tech feet sitting on top of an ISO acoustics ZaZen II platform. My phono preamp and all tube preamp are on 3" maple butcher block acoustics block on spikes. The amp is on 1 3/4" Maple butcher block on spikes as well.
Thinking of trying something different under the amplifier like iso pucks or something. But anyhow on to the point I would then have a nice beefy large 1 3/4" maple butcher block I could experiment with adding to the turntable isolation. Directly under the turntable on perhaps the rubber feet on top of the ISO acoustics platform or the entire setup on top of the butcher block itself.
Appreciate any thoughts on this. Thanks.
-Chris