Phat? Phat? Hmm... I'll make a generalization here 'cause that's what I do best, but I assume someone looking for phat bass wants the bottom end to completely overpower the mids and highs. He wants to elect Mr. Whoomp over Mr. Mid and Mr. Sizzle. If that's the case, look for something ludicrously huge... like an SVS the size of a small skyscraper - and may the audio gods have mercy on your everlasting soul.
If I've mischaracterized your request and you're looking for good tight bass reproduction to fill in below the useful range of your full range speakers, I'd suggest the Hsu or Axiom 10" drivers (EP175/VTF-2) - 10" drivers are good and punchy (there's a reason good bass guitar cabinets use 10" arrays), although the Axiom products use aluminum woofers which help tighten up the sloppy sound of large drivers (not sure what Hsu and SVS use).
Bren R.
