Which Rel sub for Audio Physic

I am trying to add a sub to my system comprising AP Sparks and a Jolida tube integrated (1000rc). Would appreciate recommendations, Rel has so many product lines, T, R, B and Reference.

I would like to get the biggest bang for the buck, $500-$700 used would be ideal.
B2 installed downstairs in my HT. Many controls to adjustment to speakers and room.
For music or HT? Either way, for $700, I'd go with a used R205 or R305 if you can find one that cheap.
This is for a two channel setup, I would like to know how HT is different with respect to Rel.
Rel recommends forward firing subs for HT and down firing for two channel stereo. I believe the down firing subs are easier to blend with the main speakers.
size of room is one of the most important factors. I went from a T-2 to a B3 with good results in a 12x12x9 room.