For middle-of-the-road choices, it seems that brick and concrete would be quite good as they have sonic characters not as over-tight bass as marble/granite, yet not too soft as wood. Acrylic is an interesting material with low Modulus of Elasticity (i.e. less crispy in sonic character).
Please see chart below, the data are from web search
It depends what you want as better bass can be more bass, tighter bass, looser bass....
But i agree you should try spike plates of different material first, they do make a big difference already. Eg fe is steel, soundcare is rubber, acoustic revive is copper, yamamoto is black ebony wood.