With my Quad 2905's I use two Rel B2 subs, started with one in the suggested corner position but realised eventually that two subs is the way to go.
The quads were positioned according the Nordost one thirds recommendation and subs using Cardas set up for full range dynamic speakers, this placed the subs slightly back and inside between the Quads.
Tonality and isolation was achieved with the Rels raised and isolated on cast concrete boots, once dialed in the results are satisfying resolving and virtually indistinguishable although my tastes lean towards the cool side.
Phase coherence is only achieved with two subs , Jim Smith's book { Get Better Sound } and subwoofer setup at the Martin Logan website have usefull info on subwoofery.
Quads excel in micro-dynamics and good subs improve the macro-dynamic aspect of these speakers. Once you hear your Quads with well integrated subs then turn them off in the middle of track you'll think someone turned out the lights.
Erik's advise is very good, and Adhendler's but I also found two subs easier to integrate and talk about the room, the best Bass in my room is with my back hard up to the back wall only due to the longer wavelengths perceivable there so you see , subs can be a tin of worms but well worth the effort in my opinion, and obviously there is a smorgasbord of subs out there.