Hmmm, Has me thinking....
You may or may not remember a while back I posted asking questions because I found an obscure Denon feature that set the sub back an additional 10db. This is a setting on its own, not the general speaker level settings. The manual mentioned something about the idea that you needed 10db less for DTS recorded material.

I wonder if I didn't bump that up to zero when I was playing with it and leave it there?

That would explain why I previously seemed to require 9 on the dial to get heavy base in my listening position before but now it is too much.

I should mention that i did do the obvious tests of turning down the dial substantially and the rattling was still there but I may not have gone back far enough to compensate for the additional 10 db I'm getting from that feature.

I'm betting your synopsis is correct in that it's now running hot but it wasn't before because I was losing 10db from that odd setting.

Thanks! I'll check this when I get home. Hopefully, this is all it is and no permanent damage was done.

With great power comes Awesome irresponsibility.