I've always been deeply skeptical about subwoofers (I'm 5.0 but mostly for the sake of my downstairs neighbour), but I've heard two really impressive implementations lately in two channel music systems. One is an REL sub (a fairly tall one - bigger than I expected) with a pair of Castle Harlechs. Sensibly integrated (really couldn't locate the sub when playing, but you felt its effect when switched off and the very bottom disappeared).

The second system used a compact and presumably quite high end Velodyne sub (all DSP-equalisation, with OSD-assisted setup and a remote control with more buttons on it than the one for my TV). The integration with the Bowers & Wilkins 804D here incredibly impressive - the already seriously capable speakers became full-on stormers and again - the sub impossible to locate aurally. No boom or waffle - just a seemingly unending stream of tuneful, deep and powerful bass. Impressive indeed. If I ever find myself living in a house I want one!!