Subwoofer rumble issue

It might be your turntable.  I had a Concept and it was super sensitive to footfalls.  I couldn't walk anywhere near it without the arm jumping around.  Granted, I have springy floors and it wasn't very well isolated, but I've had a number of tables before and after it that didn't have that problem.  I seem to remember issues with what you've described at high volumes also. 

It's surprising and unfortunate, because it does seem to be a solid and well built table.  It may have something to do with the magnetic bearing tonearm design.

There are a number of reports of this issue if you do a web search.  There are also many who love it and report no problems with it.  There seems to be a consensus that it needs to be really well isolated.

I really liked mine and wanted to love it, but it didn't work in my less than ideal setup where many other tables have worked great.  You'll notice I use the word "seems to" a lot in my post, there's obviously some supposition on my part, as I was never able to figure out how to get mine to work satisfactorily in my system and I know many others have that table and love it.  Do you have another table you can try out and see if it has the same issue?

I don’t have another table on hand to swap in to see if it is indeed the Concept that’s causing the issue. I might give the subsonic filter a shot before giving up on the Concept. I really do like it aside from this apparent issue. Time to play the DIY isolation game! 

@georgehifi you were spot on. Stylus resting on a still record then tapped the plinth and the sub rumbled like crazy. Would a subsonic filter fix this or what’s the best solution in you eyes? 

Hint: don’t tap the plinth whilst music is playing. Hint #2: that’s exactly what isolation is - a subsonic filter! Problem solved! 🤗

@geoffkait thanks for the tips 👍 I’ve ordered a subsonic filter. Hope that does the trick!