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

Buy a 18in by 18in by 1in recycled rubber outdoor paver from Home Depot and put it under the sub.  Or sized otherwise to fit your feet..  Don’t put anything on top.

isoacoustics is what you want

I have 2 REL SHO’s and they certainly don’t hop around.  You must have the volume turned up way too high and you most likely want a thumping bass sound.

I play mine at 40% volume.  If you can hear your subs they are too loud.  Play them as woofers,

I like the sound of springs over sorbothane.  I  think Herbie's is platinum infused rubber.  Many choices once you get this solved you will love the bass.

Honestly, I have to question subs playing that loud. I found that when they were set more than very low in my audio system, they collapse the soundstage and screw up the midrange and treble. They should not be heard.

I used to have 4 800 series subs, two in our HT and two in my audio system. When properly adjusted one could barely feel vibration from them with heavy bass. In my home theater playing the depth charge scenes from U-571 shook our whole house and they would vibrate a little with one’s hand on top.

From what you say, there is something very wrong.