I've heard the phenolic stands in use at a dealership. The sound there was fine, but the amp in use was muchy on the bass anyway, so it's hard to really judge the impact there...
They are adjustable, which I can say from experience is a big advantage for these speakers--they're very height-sensitive, and and adjustable stand would save me from using, for example, the books that I am currently.
The phenolic stands are a bit pricey, and may not be as inert as steel tubes filled with lead shot and/or sand, but they do seem pretty convenient. And moving them must be much easier too!