Subwoofers with your two channel system?

Thats kind of a double edged sword there buddy.

Best would be to use speakers that require no subwoofer. Get something that reaches down to under 30hz.

The sub can REALLY add alot to music, and make everything more dynamic and impactful, Ideally a 2 channel system with a subwoofer should not sound like there is a subwoofer at all, and this can be difficult.

True full range speakers eliminate that problem. If you opt for a sub, dont get cheap, extra money spent will be worth it.

Velodyne Digital Drive subwoofers are getting a good reputation for blending in real nice with music.
Also REL's subs are great for music.
Definitive Technology's Supercube subwwoofers are also getting a decent reputation of being very musical. I spent some time listening to the Supercube Reference and it blended with 2 channel very seamlessly.

I would advise buying a sub from a high end specialty store that focuses on 2 channel music.

In an ideal audio-world we all would have at least a 10x10 meter room and no wife or children to disturb us in finding the perfect placement for our loudspeaker.
So, if you have the room (and the funds) to place a full scale speaker, go ahead and opt for one which reaches into the 30 hz range.
I didn't have the room for a full scale speaker and had to compromise a bit by opting for a good monitor speaker (Von Schweikert VR-1's) and added a very fast sub (VRS-1). The integration between the monitors and the sub is excellent and you only hear the sub when you switch it off (the podium becomes smaller)!

So, adding a sub might be a good idea, but speed is important. Another thing, since you'll be running the speakers full range (no low pass filter on your stereo amplifier I hope) you want to set the crossover of the sub low. I've got the VRS-1 at 50 Hz, but the roll of is only -12 db/octave which might cause some mid-bass overlap with full-range speakers.

There is an interesting article at http://val.dorta.com/audio/archives/000109.html that talks about the use of minimonitors with subwoofers. Food for thought.