Now it's subs

I too rejected adding a subwoofer to my system until my dealer introduced me to a JL Fathom 110. One of the best improvements I could have made. I don't actually hear the JL while listening. It just blends into the system. It's plugged directly into my Cary preamp, calibrated and fine tuned. 

Subs should not be a mystery to audiophiles, and I think they have gotten a bad rap because users have under bought and not taken enough care in set up. I have been using subs in 2-channel systems with a lot of personal satisfaction for well over 10 years, and currently have three different Monitor/sub setups in operation. But. Good bass is expensive. And tricky. When you get it right it can be magical.

There are a lot of good subs for music listed above in this thread. I might add to the list the B&W PV1D for small to medium sized rooms.

some rules I apply:

-smaller main speakers require smaller sub drivers to accurately reproduce frequencies at the main speakers low frequency cut off point. 13" or 15" drivers are generally not going to be your best choice to reproduce higher bass frequencies with speed or timbre needed to blend with a great monitor. "Fullish" range main speakers that get down to the 20’s can be fortefied with some giant drivers to dig deep, but for music, speed is still more desirable than sheer volume of air moved.

-I like to match driver and cabinet materials between the main speaker and sub if possible to try to get similar voicing of sub and speaker, although I am sure good results can be obtained with mixed driver characteristics as demonstrated by many recognized high fidelity multi-driver speaker designs.

-I generally spend more on my subs than on both monitors I match them with combined - again, recognizing good bass is expensive and cannot be skimped on or treated as an after thought.

-I prefer subs for 2-channel that can handle high level signal as it imparts some character of the main power amp in the bass signal. I have in the past and am currently running one of my systems with line level rca from preamp to sub, but generally prefer high level in for 2-channel.

-I am a big fan of tuning sub levels, phase and crossover with frequency test disks, sound meter and ear. Auto Eq can be great for 5.1, but I have not heard a 2-channel auto eq that is better than good manual tuning. Not saying this doesn’t exist, just I haven’t heard it yet.

-it’s the room, stupid. The further you delve into low frequencies the greater the opportunity for room caused mischief. Getting into the 20’s and below can cause room problems to arise you didn’t know you had in your blissful 35Hz world. Care in placement, multiple subs, and room treatments can all play dividends here.

-oh, and be prepared for some surprises from what you thought was a well-matched cartridge and arm in your analog front end once your system is capable of reproducing 12Hz or lower signals.

Long Live the Subwoofer.

Most subs  are designed and designated for home theatre and are not recommended for audio systems. The exception are REL's,  very "musical" subs. Another advantage of REL, you can use it in stereo system (high level connection) and at the same time integrate it into home theatre at the same time (low level connection). You can connect other subs one way only,  either low level or high level but can't use both at the same time.
I use REL B2.  

i am a total novice about subs but just got a REL s/3 and am very pleased with what it adds to the sounds.  so i agree with JKUC above about REL for audio systems.  the funny thing is how much more expansive and full the sound is with the sub.  it does more than just add bass but fills out the sound in a wonderful way.  the thing is to make sure the sub matches the speakers: i had written REL for advice and told them what i had and they recommended a pair of S/3, but said to start with one and add another as money permits. 

I run a pair of Magnepan 3.7s with a pair of Gallo 10" subs.  The subs are run through a DSPEAKER.  It gives pretty impressive sound.  The thing I wish for most is a volume remote for the subs.  I wouldn't need one if I always listened at the same volume and the recordings all generated the same spl but I don't and they don't. Does anyone know if you can add an aftermarket remote sub volume control?