Check out the Vandersteen subs. They sounded pretty damn good, you can run two of them for added weight. (PSA: I am not a subwoofer fan, I like tight clean bass -- I know some subs can integrate well, but I have always had trouble being happy with the sound).

Here is the link to the Vandersteen Subs: http://vandersteen.com/products/model-2w-series-subwoofers

i've always had good luck with svs and they have great customer support.
when looking for a sub for my mg1.6's i started with a sb1000 and wasn't real happy with it in my setup.  on the recommendation of svs they thought the sb2000 would work much better, and it did.  it's small, quick, and integrates well with every speaker i've had thru my basement so far.

you can get a return/open box and save a little cash.  i've done this twice with svs and the subs they send are in perfect condition.  they'll state if otherwise.

if not svs i'd prob go REL route.

mark is that a dayton 250wpc plate amp on those omega's?
i've been meaning to buy one for my homemade passive sub.
sounds like you like it huh.

yeah i like the speaker level inputs better too.  

sb2000 doesn't have these.  that's the only thing i don't like about it.

Decware is to fast and real for Subs.

So, I figured out the best room size for my ZMA and the Adagio Transmission Lines are King.  You will figure it out if you stick with Decware.

I run subs with my setup, but my setup is a little different and I will give you my $0.02.
I don't care for plate amps. Had way too many issues with them. I like to run a dedicated sub amp. This could be one of those sub amps with the crossover controls on them or just an amp you have laying around. With a basic amp you will need something to control the lower frequencies just going to the sub amp. I use a Paradigm sub box, which you can find used every once in a while. Not sure whey they don't make it anymore. Its a great little box.
The best subs I have heard and have are the Decware WO32's. You have to build them or have them build for you. They are a harder build because of all the angles, but these subs are so fast they will blend with any speaker you have. I use them with my Fostex back loaded horns. They hit hard and go deep. They also double as a stand for monitor speakers or you can just sit them next to your towers. I have two WO32s for true stereo. Then on my CSP2 preamp I have two outputs, Steve added the second one. One goes to the speaker amp and the other goes to the sub amp. My sub amp is an older Adcom 545. It puts out 100 wpc. I have them adjusted just to add that lower note but on those days when I want a little extra bass I can turn them up and have windows rattling and stuff falling off the walls. These subs are great.

I have an older Velodyne dual firing powered sub.  While not exactly considered audiophile, they have always had a solid reputation.  Mine has the 10" front, and 12" down firing with line level and RCA connections, high/low pass filter, phase in/out switch, and fully adjusted volume and crossover.  Back 15 years ago, I ran me like $1400.

I mainly used it to only hit the really low frequencies, and never ran it past 3 or 4 on the volume level.  I have to say that it is truly a beast.

I agree with Jeff1.... The front firing is better for audiophile listening.  But...If you plan to use the sub in a home theater setup, the down firing is what will give you feeling of a powerful explosion, gun firing, and room shaking experience.  

I had a small Rel Strata II a long time ago.   It was a great sub; probably the best sub I've ever had.  But, Rel is not cheap.

RJR...a little experimentation with placement (and crossover/volume) and I have no doubt you'll get the REL to blend seamlessly.  REL has a placement guide, but IMO they are suggestions, not rules.  I ended up with the sub on the outside and slightly behind one of the mains.  

Your ears will tell you if something isn't right.  Better yet, they will let you know when it is.  

Have fun and let us know how the sub works out for you.