Subwoofer insight.

Roll the low end out of the mains with a simple cap XOver, say 80-100Hz.

Check out  http://www.ielogical.com/Audio/SubTerrBlues.php for integration advice.

I ran a single 10" w TC-50s in 16 * 30 * 7 to good effect.

Adding good low end increases realism beyond all proportion. The operative word is good. As a composer friend opined on hearing my system "Every other subwoofer I've ever heard just boomed!"

Were I you, I'd opt for something with continous phase and a polarity inversion. Without them, integration will be problematic at best and impossible at worst.

Check out 2x ML800x as control is far more extensive than the RELs

Ignore everything millercarbon says.

As they say, you only know what you know. So far I have been impressed with my results with the RELs.  I suppose I would need to move outside my comfort zone to take the advice what many are saying here.  I do feel like the air is moving.  The only thing that seems bothersome is having to readjust the volume based on what is playing.  Sometimes to boomy.  Seems to be album to album.  Not song to song. Maybe I am doing something wrong but only had them 1 day.  

For Elliotbnewbcombjr, I think what you are saying is to come out of the 4ohm taps that the main speakers are coming out of to the high level input of the RELs.  I could easily do as I have female banana spades on the REL high level cable.  However, REL says best to come out of 8ohm or 16 ohm taps.  Appreciate your insight.  

I researched both REL and Rythmic.  I think either one is a good choice, but I think the high level connection designed by REL is genius.  The high level connection allows the subwoofers to play more like woofers.  The key is calibrate then to not play too loud.  If you can detect them you either have the volume turned up too high or you need to calibrate the crossovers to blend seemlessly.

I have a pair of Paradigm Prestige 85F towers.  Originally, I was going to buy a pair of REL T9i"s, but went with a pair of REL S3 SHO's.  Glad I went with the S3's.  No matter what you decide, go with a pair.  Much better sound than just one.  I would not get anything smaller than the REL t9i's.  Right now playing two bookshelf speakers is lacking an enormous amount of bass.  You could also purchase towers unless you have a space limitation.  If you are limited in space and have to settle for bookshelf's, spring for a pair of the REL S2 SHO's.  I can put you in touch with someone who can surprise you with a great price.  This was how I could afford the REL S2's.

Why is a subwoofer high level connection genius?
IMO, it's as hopeless as trying to mate a whale with a mackerel.

Rolling the low end in the mains and properly integrating phase and level therewith blows the doors off padding the low end unless one is very, very lucky.