Sub-Woofer?

I've got a 'classic' system; Rega Planar 3, Sonographe SC1 pre-amp, Counterpoint SA12 amp and Cabasse Galiote bookshelf speakers.

I'd like richer bass and am planning to add a REL sub-woofer. I am looking at a T5 on Ebay. Any thoughts on this? Good idea? Bad one?  any caveats?

All input appreciated!

Gasbose