My personal favorite for a $500 sub remains the Rythmik FV12. And in a small room like the OP's, the FV12 will have ample output.
Similar to the HSU VTF-2 MK4 (which is a very close 2nd place in my book due to being just a wee bit more expensive), the Rythmik FV12 allows you to "shape" its low end response with filter controls, which can be a huge benefit in a small room that will have a lot of low end room gain and strong reflections. The FV12 is also "unbreakable" since it has good limiters and compressors for high volume output. You can't fill a very large room with reference level output with the FV12, but you can crank it up as loud as it will go without worry, which is nice
Transient response is excellent for this price range and the basic frequency response is nice and linear. Extension is very good and is almost flat right down to 20Hz. You won't quite troll any infrasonic depths with the FV12, but it goes remarkably low for such an inexpensive sub.
So honestly, my feeling is that for $500, you cannot do better than the Rythmik FV12. That said, HSU's VTF-2 MK4 is an extremely close 2nd. And if you can buy the VTF-2 MK4 and avoid the shipping charge, then it might even be the better deal price wise. It certainly will be if you get any sort of "B-stock" or other price reduction
SHARP Aquos LC70LE640U
Denon AVR-4311ci, Emotiva IPS-1
APC H15 Power Conditioner
PS3, Wii, XBOX 360 Elite Slim
Dell Inspiron 660 HTPC
Def Tech BP7001SC, CLR3000, BPX, BP10, BP2X
SVS PB13-Ultra on Great Gramma
Explorer 4250 HD cable box
Yamaha RX-V1800 Onkyo M-5140 amp
Xbox 360 Elite, PS3 Slim
Dell Vostro 400 HTPC
Def Tech BP7006, CLR2300, BPX
SVS PC12 Ultra & 20-39PC+ on Subdude HD
I would look into the outlaw B stock https://www.outlawaudio.com/mofcart/bstock.html.
They have the outlaw plus at $499.00 with free shipping...this sub is a beast, it can filled large rooms with no issue at all.