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.

I recently picked up a Velodyne DD-12 (part of the Digital Drive series), and it has been the most significant upgrade that I have done. My gear is more than capable of reproducing clean, accurate deep bass - Thiel CS3.6's, with Theta Enterprise monoblocks. But my room is an acoustical nightmare, with no low frequency reinforcement and numerous peaks and valleys in the low frequency response. The ability of the DD-12 to analyze the low frequency performance of your system, in your room, and then to fine tune the sub's performance to correct for it, it nothing short of revolutionary. What a tremendous differnece it has made! As an end result, my system is more accurate, and I believe more musical. I bought the 3.6's because I didn't want to have a subwoofer. And in most rooms, there is no need whatsoever for a sub with 3.6's, but in my room, adding the DD-12 made a tremendous improvement.

As for musicality, the Digital Drive series employ Velodyne's servo technology (which keeps the cone from continuing to vibrate after sounding the note), and even give you control over how strong the servo response is. This means that you can have one preset that is geared towards HT, and one that is tighter, and geared towards music. Or, do like me, and keep it all nice and tight. With a 1,250 watt RMS amp, my little DD-12 can keep up with the big Thiels quite nicely.

Highly recommended.

Tom.

Wow, that's some recommendation. What do those things cost?

My DD-12 cost me $2,100.