Best isolation under a big receiver ?

Timtim, there's not hard & fast rule for isolation. different solutions work for different people. For example, I have my amp sitting on its factory feet but I've got the amp sitting on my home-made amp stand which is a sandwich of hard maple & granite.
What I might also suggest is that you open up the amp & apply vibration damping sheets (can get these at Parts Express or a local car audio shop) to as much of the internal metal chassis as possible - vertical inside walls, underside of top plate, any exposed metal on the bottom plate. This damps the chassis & prevents it from ringing.
Try different isolation platforms - hardwoods are better (hence my choice of hard maple). Granite is also good but make sure that it is not suspended in air where it can ring (as it rings pretty good!). Damp the underside of granite with a cork sheet.
FWIW. YMMV.

Thank You for your response

I have been researching this on AG since first reading this thread!

If you don’t want to spring for a Symposium platform. I like a hard maple platform with cones under the component. I’ve found it opens up the sound while enhancing detail.

Amazing that it helps that much

My background is live sound. The fact that they make combo amps with the amp inside the speaker cab and assuming these principles apply to all amps in general really baffles me. They would design and sell these combos for thousands without properly dampening the amps. We often place our amps on top of our cabs. I play bass guitar and to think I have been potentiay killing my tone does not make me happy.

Maybe with live sound it's not as important but after researching these principles and the lengths audiophiles go through to dampen with the results that are gained really opened my eyes to this issue!