There are some great suggestions in this thread but, not surprisingly, nobody is mentioning the exquisite Perlisten line. Perlisten has a low profile on Audiogon, perhaps because the company rarely submits its products for review. But make no mistake, these subs are pretty incredible, deep low-end extension (12 Hz with built-in bass boost), effortless, transparent transient response, extraordinary detail.
They're not cheap, starting at around $2000, and don't offer a ton of bells and whistles, but they cover all the bases most important to audiophiles -- especially SQ. IMHO, they're definitely worth at least seeking out.