Sclapp,
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.
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.