Experiencing great results using 2 REL B2 subs with Quad 2905's. After much experimentation seamless intergration was found in my room by placement only about 3 feet behind the inside edge of the Quads and toed inward.
Room resonances were controled with cast concrete blocks under the Rel's (aprox 25 kilos) and about 5" thick. Cones are placed under the front feet resulting in a backward tilt.
A pair of REL B2 or B3 here on can be had well under your budget and 2 subs are better than one in my subjective opinion.
I havn't had any experience with the subs in your enquiry and don't doubt their application, but I've found subs are like real estate ,location , location, location .
Keep us posted on what you end up with and the results, very interesting.
Regards.