which sub to get

A benefit of using even one powered sub is the potential to offload a lot of
heavy work from the main amps and speakers so that alone will often result
in better sound even if there is still likely room for improvement by adding
subs. In many cases the added benefit Of more subs may be marginal
enough to not matter to some. I've run with a single sub crossed over
above 50hz in the past and been quite satisfied. Although bass levels
varied at various room locations. Also I know technically imaging and
soundstage would be even better with two well placed subs but practically it
sounded spot on as needed in the sweet spot.

I just purchased a SVS SB13 Ultra and it adds to my musical enjoyment.

Some of these comments are just canned responses that probably don't apply here. OP specifically says it's for near-field listening in a small room, so I would think that he should be able to get good results from 1 sub.

I think there's a great case to be made for multiple subs, but in MOST rooms, especially if mostly near field listening, you really can easily get away with one...if it's a well designed sub like REL makes. "Aural cues" that determine soundstage accuracy aren't in the bass domain really, and note that a REL sub when utilizing the "high level" signal from the power amp won't relieve the main amp's task, but will certainly SEEM like it is. I have skinny tower speakers with a front firing REL behind one speaker and it loads my large-ish room just fine...a lucky acoustic profile in this room helps everything, and I do have to adjust the sub level here and there.

WOLF_garcia wrote " I do have to adjust the sub level here and there." I totally agree. It is absolutely important to
to set volume precisely. Most audiophiles set volume too
high, presumably because they want to hear strong, powerful bass all the time. A properly adjusted sub do not call attention to itself all the time; only when the music demands it.