Tho I don't have exper w/ those, perhaps Klipsch subwoofers would be suitable. They have choices in the $300-400 range.

Emotiva Ultra Sub 12 for $429 delivered.

If you are not doing HT as the sub is for a music mostly system the 10 will probably do as it will reach down to 25 Hz. Just set the crossover frequency on the Ultra Sub 10 to 50 Hz and it should mate well with those RB-51s which rolloff at 50 Hz.

Try
Boston Acoustics 10" 100-Watt Subwoofer (CSUB10B) looks and sounds awsome, I think it's around 400 maybe less.

is underperforming (32Hz -3dB), underpowered (100 watts), ported (sealed is better for high fidelity), with questionable BassTrac circuitry (is it servo or what?) and does not have favorable price/performance for high resolution low frequency reproduction for critical music listening. There is a reason Boston Acoustics is trying to get rid of them for $200 and you can find them elsewhere for $164.

I've had it for a while, I've been watching blu ray's and music in 5.1 audio (had the top floor or my 3 storied home shaking) it sounds amazing. The reason thier selling them so cheap now is I think they got a new model coming out, but if your looking for a reasonbly priced bass with good quality sound go with this.

For this budget I would build my own sub setup for biggest bang for my buck. The required lumber from Lowes would run you less then $30. A nice JL Audio 12 in a ported enclosure would do the trick. Of course this assumes that the receiver has a built in crossover. If it does then use it's advertised low pass crossover point as a reference to determine the port requirements such as diameter and length and tune it to match the crossover point and slope.

Hey, no one said you have to compromise performance because your pockets are tight. You just need to be able to go the extra mile to achieve the desired results.

would be as cost effective and yield better results for a music only system.

http://www.customanalogue.com/sub_index.htm

I'll be building this without the amp and amp chamber for use with my preamp which has bass management, a power amp and an Elemental Designs eQ.2.

Hrmm, now that looks like a fun project