Adding a sub woofer?

Thanks Rusty - helpful. And thanks all for your comments - plenty to dig into here, lots of homework. I appreciate all the thoughtful ideas and suggestions.

SVS SB3000! I have this and a Sunfire HRS12 that seem to play very nicely together - both musical, accurate and DEEP... 

Hello,
I just added a pair of REL T9s to my system. The T9s are active/ amp driven down firing with a passive front firing driver. The T9i or newest model is the opposite with the front firing driver is active/amp driven and the down firing is passive. I feel the front firing is a better design for music.  I like to use a pair of subs for the front. If you choose to go with a speaker terminal connection or speakon please call the manufacturer for setup. Differential balanced or mono blocks require a special termination due to the type of ground connection when connected to the speaker terminals. ( Very important so you don’t damage your other equipment.) I also prefer a sealed enclosure for music instead of a ported enclosure. HSU has a unique enclosure that lets you block the ports to turn the ported enclosure to a sealed one.  Ported or holes in the enclosure go down deeper in frequency but the bass has less control. Great for movies to shake the room. Home theater works better with the ported enclosure to shake the room and can cost less per lower Hz frequency.  I am doing two subs L/R for the front and a ported enclosure for the surround sound connected to the LFE /RCA connection on the surround sound receiver. Also sealed enclosures are usually very fast to help match to the front speakers especially RELs. Since you usually have only one active driver per sub I believe it sounds better to have two to match or add to your left/ right front speakers. Also this is much more balanced since you are setting the sub on the side of the speaker you are trying to improve. By adding more subs you can meld the subs to the system easier since you can turn the volume/gain on all the subs down to prevent the boom. That is why I like a separate ported sub for movies usually on the side wall in front of the seating area. I will probably get two more T9i RELs for the front and move the T9s / down firing to the rear of the room later. Sorry for the length of this post. Subs have a lot of detail to explain. When you decide to get a sub look up sub crawl to help set it/ them up. 

I added a pair of HSU ULS-15 II subs to my Cornwall IV speakers. I am running the CW full range and bringing the subs in @ 35 Hz. I can very much recommend the HSU  brand, these things integrate perfectly with the main speakers and are unobtrusive unless called upon for duty.

Oz

After moving some years ago, my listening space became smaller and the Vandersteen 2CEs I had were no longer a good fit. Replaced them with Kef LS50s and enjoyed the change but missed the low-end of the Vandies. Finally bit the bullet and got a Rel T/5i sub to provide the missing bass.

Using speaker-level connection and corner placement, as recommended by Rel, I dialed everything in using 1/3-octave warble test tones and SPL meter.

Adding the sub noticeably increased the soundstage (width & depth) and made for a much more enjoyable listening experience. Highly recommended for anyone with small main speakers.