Tbg, I don't believe Acrylic by itself is necessarily bad sounding. All materials have an inherent sound when used as a base, whether Corian, marble, granite, wood or MDF. The component base rack was a step up in their technology and was wider than the original. Also, the absence of any shelf material on the bigger rack probably played a role no matter what shelf material might have been there before.
I thought the original Stillpoints rack was the best isolation I had experienced with the original acrylic shelves, so I was taken aback at the improvement gained from switching to the stacked component base rack. As always, context plays the lead role in our preference and experience. Their OEM footers also made a permanent and positive difference under my MAXX 3's.
As with anything its best to try and compare when possible.
Grant
Shunyata Research