Current Inexpensive sub that can get it done cheap

Not sure how big your space is, but I othewise second the Paradigm Servo 15 recommendation from above posts.
Other that that, for God's sake, get some matching speaker in your barn system, will ya?!! I just don't understand people trying to put together a bunch of mis-matched, otherwise good quality speakers, and expecting coherent, believeable, hi-fidelity sound! It's like using a different left and right speaker - you wouldn't do it!!
But I think this topic has been beaten like a dead horse.

I think Tobacco is right get a 8-9 sub on agon or ebay from someone with a good track record. I would look at REL Velodyne and JL Audio

Well, inexpensive means buy from the secondary market (preowned). The prices I see on gear in good condition 8 or 9/10 and can be almost 50 to 60% of list. So of course, you get better for less. Sellers on this site are pretty cool. I just bought a Velodyne for wayyyyyyy less than list price, and its new in box. I made my decision based on reviews, looking at posts, and asking my fellow audiogon folks. Personally, I didn't audition any with a retailer because it means I would have them show it to me and then turn around and by it somewhere else. But that's just me.

if 'snappy' means clean, fast, accurate you want a sealed or servo design, though you will give up a bit of maximum volume and/or extension.

you might consider some of the internet directs: epik empire (2x15", sealed) or legend (2x12", sealed), or rhythmik F15 (1x15", servo).

i have a single empire in a 3300ft^3 theater, and i am quite pleased. it stays ruler flat to 20hz in my room. a 2nd will makes its way here before too long. there is absolutely no comparison with the outlaw lfm-1ex (1x12", ported) that it replaced.

to help alleviate the effects of standing waves, 2 smaller subs is almost always better than a single. also, you might want a sub eq (SVS as-eq1 / velodyne sms-1 / antimode 8033)...they can make a world of difference. i have the SVS and it was worth every penny.

If you can find a Definitive powerfield 12 or 15 it would definately fit the bill.
I have had a 12" for about 10 years and I am listening to it as I type this.
Built to last, it also sounds very, very good, just not as fast as the Paradigm servos.
Sounds really good on movies and music as well.

I would second the Paradigm Servos no matter which model.
IMHO they are exceptional for the money.

If you don't mind buying used, there is a Paradigm Servo 15a with x30 crossover for sale now that would definitely fill the bill. No association with the seller.

Well engineered ported designs as good as sealed designs IMHO.
Each has its tradeoffs -
Sealed boxes have issues mores on an issue with standing internal standing waves being feedback to the speaker cones. Ported designs exhibit other possible (arguably) tradeoffs. I’ve heard designs of both schools that sound excellent. All rooms exhibit ‘modes’ – they just occur at differ frequencies (they all will fall into the subwoofer range). Different speaker drivers end to work better in different designs depending on several factors. A major one is the ‘Q’ of a driver. The bottom line is that the driver needs to match the box design. One is not necessarily better than the other. Room modes and reflections also occur at all other audible frequencies. There are quite a few places on the WWW that will explain how to calculate them.

Thanks to all who replied so far! I did fail to define my use of "inexpensive" as was pointed out. I would say suggestions in the $1000 and less would be in my range.
This sub will be used for 2 channel music only, with the cdm7's. My room is 24 x 12 with 8 foot ceiling.

Madisound has either a 10" or 12" powered subwoofer at a reasonable price. Check their website. Highly recommended.

Be cautious when buying inexpensive subs as they tend to produce more distortion and the distorted 1st and 2nd order harmonics can compromise midrange sound quality

If your main speakers are ported look for a ported sub, if your mains are sealed look for a sealed box sub ... if your main speakers have steep 4th order crossover slopes look for a sub with steep 24db crossover slopes, if your main speakers have 1st order crossovers you need to look for a sub that will work with that gentler first order slope ... all this will make blending sub with mains easier

More important than the name on the box is correct room placement ... setting of crossover point ... set it to high and the sub will sound slow and muddy ... try setting crossover 1 octave above main speakers -3db point and then equalize volume of sub to match main speakers

If your looking for slam stick with a 12 incher and don't worry about the room being to small as the wave must go back and forth to complete one cycle ... so a 15 foot room could handle a 30 foot wave or 28hz note

Find a sub with low distortion specs that has similar build characteristics to your main speakers and let the shopping in your price range begin

Now can someone please explain to me why I need 3000 watts to drive a sub to 121 db @25hzs when my mains speakers play at 90 db ... TIA

The new REL sub is less than $800 retail. Small little thing. I think it's a small T Series?

Depending on your definition of cheap, check out partsexpress dot com and look at the Dayton sub kits. From my understanding the sound decent and can really pound it out for the low cost they are asking.

Any of the Martin Logan subs will work for you, a used Grotto would probably be the best value.

Ballan-
the new rel g-series are going to start between $3500-4000 for the G1. The following g-series will be released sequentially in about 9-12 month increments. Not the "cheapest" of subs.

Defcon3-
I have a Rel T2 that I am pretty happy with, it does double duty in my system for two channel listening and ht for movies. Being able to have separate gain controls for the neutrik and lfe inputs allows me to have well integrated low end for music while at the same time having it turned up more for the movies. It won't play the lowest or the loudest but for under a grand it does a good job.

check out the ad on here for a pair of Epik Sentinels.

Kgturner is right. Having said that . . . Hsu Research.

you're gonna have to define "inexpensive" for us.

REL has a new sub coming out within the next month that sounds great. I think they will be debuting it at RMAF.