Best way to keep REL sub from “hopping around” at high volumes

Replace it with an open-baffle dipole sub.

op

now, the right tent material offers very useful dampening of high frequency energy! 🤣

i do hope you sort out your sub dancing issue with blu tack!

+1 @dekay That's what I use as well.  However, I have wood floors so for the most part they stay put - there are few tracks that can move them. Sounds like you have slippery floors, so it may be at least worth a try.

My DIY…Herbies Audio Labs “thin grungebuster discs”. Very grippy, not gooey, messy or marring. About as thin as a business card and they keep that bad boy in place.