My rack also has MDF shelves. As an experiment, I bought a 3x20x20 maple block from Tonys Woodshop (tonyswoodshop.com) and a set of the 99lb Mapleshade Isoblocks to go under my Teres 255. At approx. $125 total cost, this is easily the biggest bang for the buck tweak I've made to my system. I'll heartily encourage trying it. Sonic improvements include a general lowering of the noise floor cashed out as blacker backgrounds, slightly better instrument focus, less lower frequency transient smearing, and most surprising to me, the soundstage lifted several feet.
Denser woods may very well give further improvements. The higher-end Teres models all gain mass via denser cocobolo or rosewood. I'd try the maple under your table first to learn if pricier solution is worth the additional expense. If so, you can always use the maple for an amp or a cutting board.