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.
