What to look for in a sub?

I've been pretty happy with my HT setup using a Yamaha RX-V1, a mix set of speakers from Definitive, Paradigm, Acoustic Research, and a really old Fosgate subwoofer. (Most of the fund goes to the 2-channel audio setup) I am moving to a new house with a smaller living room, and the sub is too big. (30"W x 30"L x 15" H) I need to find a replacement sub that will fit into the new living room. What criteria should I look for in a sub? Also any suggestions for a good sub (new or used) in the $800 range would be greatly appreciated, active or passive. (I currently have a Marantz 50w monoblock driving the Fosgate).
The Earthquake Supernova 12" subwoofer can be had for around that price, maybe a smidge more new, and it makes a very hard hitting, very low playing, excellent blending subwoofer that will put out lots of low bass output at volume in even larger rooms if need be! It's better than just about anything commercially available in that price range for shear output and integration/excitement factor!
Good luck
My recommendation is the ACI Force subwoofer. An outstanding little sub, with compact dimensions, that is flat to 20Hz, +-3db. Its performance is identical to the giant-killing Titan II LE (also from ACI) except for slightly lower output. Pricing starts at $749 for basic black to around $850 for real wood veneers. Check out specs at http://www.audioc.com.

Energy makes an awesome/tiny 8" sub for next to nothing. Paradigms are nice too. Any bigger and I'd consider Hsu. I hated my sunfire-buzz/humm and it danced around when worked hard. Pinnacle Baby Boomers are supposed to be really nice and tiny.
You will definitely benefit from a larger amp for the sub. 50 watts won't cut it. Janis Audio systems reccommends that by cutting all frequencies below 100hz you virtually quadruple the available power. Bass is power hungry. If your main speakers are getting 90 watts, your sub should get at least that but probably way more (depending on efficiency). Rule of thumb: you need to double your wattage to gain 3db, 10db increase will double the volume. Therefore, you need to approximately gain 9 x times the wattage to double the volume!
Thanks to all for all the information. Elevick, are all the subs you mentioned active or passive? I checked my little monoblock last night. It actually outputs 250 watts, not 50 watts. Thanks for the info on the Sunfire. It was actually on my list. Now I can take it off!
Check out the Mirage BPS-400 sub. (2) 12 inch active radiators with a 400 watt amp. It list for $1300, but Tweeter is selling them for $799. I only use mine for HT applications, but it does 'rock the house' very impressively.
I'd look at the REL Q150E. Small (12" cube), musical, good for HT and connectivity that no one else has. You can run hi and lo level inputs at the same time with cutoff set for hi level and full passthru for LFE (.1) info. Can be had new for under $800.

Another good one is the MJ Acoustics Pro 50. Another 12" cube, goes down to 15Hz, but you don't have the connection options of the REL.

I am really happy with the NHT Subone. $800 msrp, street price is less. It has line level and LFE connections that can be used simultaneously, and a controller box with the level, hipass and rolloff adjustments. This takes the electronics out of the vibration-filled speaker cabinet and puts them where you can reach them. Integration is seamless and the sub is great for music, too. It has a 250watt amp from Sunfire (no hum at all. BTW, if your amp hums, it's probably broken). I was reluctant to add a sub but I'm glad I did.
In this price range, if you're looking for good overall performance and smaller footprint, i'd suggest one of the cylindrical models from SVS (http://www.svsubwoofers.com) or HSU (http://www.hsuresearch.com). These are internet-direct companies that provide EXTREME bang for the buck, very good customer service.

I own a pair of SVS 20-39's with the Samson S1000 outboard amp. VERY satisfied. In your price range, you could buy a passive from their line or a powered cylinder.

I would urge you to investigate both companies and read the comparison reviews on thier websites.

Good luck,

I've been through a few subs recently, Sunfire, HSU, Velodyne, and here's what I settled on: A Pinnacle Baby boomer for music crossed over at 80hz via a M&K external passive filter. I have the 10" cube located between my Axiom M60's, together they sound awesome on music.

For Home theater in addition to the Axiom's and Pinnacle, I have the Earthquake SuperNovaIV 12" sub placed right behind my couch and oh my God, it ROCKS the concrete foundation on movies.

In short, the Pinnacle is great for music, and the Earthquake blows away everything else I've tried for HT. And for a 15" cube that is amazing!

Good Luck, let us know what you ended up with!

I think the most overlooked objective when shopping for a sub is getting the right design for your listening room. You first need to decide whether you want a front-firing, downward firing, bipolar or other. Then you should shop for the best sub in that catagory.
In my smaller listening room, front-firing subs just overpower the room. I need a downward firing sub (HSU) to spread the bass out more evenly. I found this out by trial and error. I started with a good sub (an M&K V-125), which sounded good in the store, but sounded less than optimal in my listening room. My room had uneven bass responce that didn't sound very good this way.