For accuracy, Rythmik Audio subwoofers are good. Epik is another good subwoofer brand, so many others like Hsu and Velodyne. Rythmik Audio uses a different method to create a servo subwoofer compared to traditional methods and they are cheaper. Velodyne makes servo subwoofers and they are the best at servo subwoofer.
Rythmik Audio is tailor for the DIY community, but they build their subwoofers too. There are other brands that just make woofers for the DIY community. A DIY subwoofer has more output than commercial subwoofers and can easily be designed for any setup. The DIY route for subwoofers is the best for your buck. The DIY route gives you the option of an infinite baffle subwoofer which commercial will not. The benefits of an infinite baffle subwoofer is accuracy and very, very low distortion. Also it can hit low depending on the resonant frequency of the woofer used. The following is good example for woofer for an infinite baffle.
AE Speakers IB15
http://www.aespeakers.com/drivers.php?driver_id=8
You can build other type of subwoofers instead of infinite baffle. There are sealed or acoustical suspension, ported or bass reflex, and bandpass. For musical, sealed is better. Vented is good for output, but it penalizes accuracy. Bandpass is output, but worst accuracy compared to ported. Bandpass is good in cars because it does not have to fight against the problems inside a car. Putting a subwoofer inside a car creates a complex bandpass box, so making a bandpass subwoofer to be used in a car provides a more predictable output.
For your setup I suggest either a Epik Legend or a custom build with Rythmik Audio servo subwoofers. For a custom build from Rythmik Audio, I suggest a dual driver using two GR Research model GR SW-12-8ohm with A370XLR Servo amplifier in a box that is between 3.2 cubic feet to 3.8 cubic feet. You could select Rythmik Audio original woofers with their amplifiers, but they use aluminum cone that makes it too heavy to handle a high crossover. The Epik Legend uses two 12 inch woofers in a dual opposing setup. It cancels out vibrations, so you just hear the sound. It uses a sealed type of subwoofer. You can do the same to the custom build using Rythmik Audio products.
The difference between Epik and Rythmik Audio is that Rythmik Audio uses a servo subwoofer and Epik does not. Servo subwoofer decreases distortion, so it is literally 3 dB louder. Rythmik Audio gives you control, so you can fine tune the dampening and the bass extension.
The problem using one subwoofers for music is the bass that supposed to come from left or right is not coming at those channels. It is better to go with full range speakers that can reach down to 20 hertz, so you do not have to worry to use a subwoofer. A subwoofer is good for home theater or movies, but not for music.
I disagree with moonboy403 because integrating a subwoofer with speakers is not hard. I have done it with the components that I already have like the computer and Audacity. To integrate a subwoofer for speakers do the following.
1) With a computer generate pink noise that is about 15 to 30 minutes long.
2) Set the subwoofer volume to 0 and set its crossover at its highest. Also set phase to 0 degrees.
3) Play the pink noise to all speakers at good level, but not too high that it hurts your ears. While it is playing, adjust the subwoofer volume or gain. Stop when you hear the subwoofer.
4) Set the crossover to the lowest since we know the highest baseline of what the subwoofer sounds at the highest crossover setting.
5) Starting from the lowest crossover setting, adjust it until the subwoofer starts blending in with the speakers. You may have to adjust the volume or gain of the subwoofer after it starts blending in.
6) Stop playing the pink noise and play music with bass content. Fine tune the crossover and loudness of the subwoofer. You can fiddle with the phase to see how it affects subwoofer with the speakers.
7) To polish the integration, use a real time frequency analyzer to make sure there is no gaps in the frequency response.
The subwoofer should not be any further than 6 feet from the speakers when the crossover is high. If one speaker is further from the subwoofer than other speakers, that speaker will sound puny. It is best to place the subwoofer in between the speakers to integrate them well.
