Before I bought my Goldenear ForceField 3 - I did quite a bit of research.
Your choice will also depend upon the primary use being Home Theater or Music. Mine was primarily to provide TRUE subwoofer "foundation" for some bookshelf-size speakers on stands.
I also decided that an AMPLIFIED sub was the "way to go" for me. At the time, I had not upgraded my Power Amp, and the Amplified Sub off-loaded the Low-End "grunt" Amplification duties to a purpose-built Amplifier that is specifically matched to the driver(s).
What I REALLY wanted was one of the T-Series from REL. I heard a few at a reputable high-end dealer and was truly impressed - it "Vanished into the backround", and there was a HUGE difference without it vs with it. However, a bit too pricey for me at the time.
Another good "budget" line that I've heard good things about is Hsu Research. Never heard one personally, but people who I trust have had good things to say about their products.
Of the units that I was able to audition LOCALLY (When I'm about to spend at least half a C-Note on ANYTHING, I want to see, touch, and in this case HEAR one BEFORE I buy. Shipping a sub could get expensive.) GE Forcefield provided the best "bang for the buck". It has a good warranty and according to local dealer, is dang-near "bomb-proof" quality and durability-wise. GE has designed their own purpose-built amp / crossover boards with really solid construction / componentry (much like Emotiva). With some adjustment - my FF7 is exceeding my expectations so far. I probably STILL would have preferred the REL (tighter, more controlled for music, harder to located within the room) but naturally, more $$$
You won't go wrong with their Monollith line. The specs and construction are solid and real world published numbers don't lie. You will never get the output of a 15" from a 10", but at that price you can do 2 and still be ahead of a single 15". As with any subwoofer, you will need to know how to properly dial it in with the rest of your system for maximum performance.
Magnepan MG1.6qr HSU Research VTF-2 MK3 2x Parasound A23 Parasound P5 Cambridge Audio DACMagic Plus Sony PS3 x4 @3foot runs of 10ga Belden speaker wire. GRADO SR80e on-ear headphones iMac as music server (2009) Panamax 5510 w/) ISO transformer LG 65" B6 OLED display