Try choking off !/2 of your bass reflex port to change "mechanics" of the bass----Sorbothane is "dense" & is for heavy shock---Try mattress foam, styrofoam or "beach" sand which is rounded grains (not crushed sand that compacts)---dust cover is a reverberation chamber, & if it's bass radiation---turn your sub a little left or right off center---I've never been able to operate a turntable if a speaker is pointed at it---especially if you're tracking under a gram---& the cover causes reverberation----You really need physical separation---or a radiation shield----like a piece of whatever you got---like piece of plywood---or cookie sheet-L-OLOL
Thanks for the tips. Things are mostly under control. No separate sub to deal with; the speakers are full range. The turntable is on a wall mounted shelf. I always play it with the cover up (although it doesn't matter when streaming). I have tried the speaker ports open and plugged and use them whichever way is better. I measure the room and use 1/3 octave eq to smooth frequency response.
Through all of my moves I have found it usually takes about three iterations of tweaking to get things where I like them; I have a couple to go (tweak, listen, tweak, listen, etc.) After that, then I leave things alone.
