I have used a number of items under my Krell monoblocks, both individually and in combinations (bearings, spikes, amp stands, vibrapods, etc), and, in my system, just heavy brass cones/feet perform the best. There is more seperation and air around elements within the soundstage, and a much tighter focus to the sound. Bearings had less air and the elements in the soundstage were less dense; vibrapods were even less so.
YMMV
Greg