Mass loading via sand dates back far more than thirty years, to the 50's, when it was used in the huge bass reflex enclosures of loudspeakers such as Wharfedale, their large unbraced plywood panels resonating like crazy if not so damped. The British have in general disapproved of ml, feeling that mass merely absorbs vibration and releases it later in time, doing more harm than good. The Brits favour low mass/high rigidity structures, which moves the resonant frequency of objects higher, where (they feel) it does less harm. I mentioned the Bright Star Big Rock only because the OP asked for a "mass loaded material" to put under his amps.
Mass loading has definitely fallen out of fashion (including with myself!), but that's what he asked for! I would instead use an air bearing (the original Townshend Seismic Sink, or a DIY version thereof) for vertical isolation, coupled (no pun intended) with a set of roller bearings for lateral. A product like the Minus K would be even better, but at around $5k for a pair and far taller than 4", is probably unacceptable to jazzonthehudson!