Which sub to get for HT NHT speaker system ?

Hello folks.

I currently am running two SW2Si's off of a a SA-2 amplifier, with the following speaker config (7.1 setup):

Mains: 2.5i
Center: Audiocenter 1
Surrounds: VS-2a
Surround backs: Supertwo

I think the SW2Si's are decent for music but they suffer greatly in home theater applications. I would like to move these upstairs into my 2-channel setup and get a sub that can deliver more bang for movies.

I've thought about the SubTwoi and SW12 but I am not sure how much of an improvement that this will give me. I am open to other manufacturers but am concerned about how well these will mate with my other NHTs.

So what will offer the most bang for my buck given my setup?

I'd say budget is around $750 to $1000.

Here is the rest of my setup:

Aragon Stage One Pre/Pro
Aragon 8008x5
Aragon 8008BB

Thanks for the help.
I have a similar setup running (2.5i's, AC-1, 1.5's, 2-SW2p's) and I also have a SubTwo and SubTwoi. At one point I tried the SubTwo in my system and although it is a very good subwoofer it just didn't give the impact for movies the way the SW2p's do (I believe this might be due to the porting of the SW2p's), the SubTwo and SubTwoi are tight musical subwoofers. If you want the impact that your SW2si has with greater power you might want to look for a SubOnei which has a 12" driver and 250 watt amp with outboard controller and is the same size and look of your SW2si. These are just based on my NHT experiences but when it comes to subwoofers personal taste really plays a big role so you might find you prefer the sound of some other brands like Velodyne, etc... Velodyne also offers the SPL1000 which is very good and can be found used for about $500 - $800. The best thing you can do is find a few good mid to high end stores in your neighborhood, grab a dvd your familiar with and go have a listen to various subs, you may find something you have to have that you never thought of. Good luck on your search.