which sub to get

Neither. Rythmik F12G.

Two smaller ones.

SVS SB-1000 is a nice sealed sub. I used one with Harbeths and they would blend nice with any speaker. SVS has a 45 day return policy,so if you don't like it you can return it with full refund.

It doesn't matter which sub, just use two.

FWIW:
In my small (11x13) dedicated listening room the main speakers are a pair of Sonus Faber Liuto monitors As an experiment, I added a couple of Andrew Jones designed Pioneer SW-8MK2 subwoofers. I'm quite pleased with the results. The ROI on this combination has exceeded my expectations by a large margin.

YMMV of course.

Get a REL and you really only need one.

You may only need one sub but it depends. It is a practical compromise though. Two are really needed for optimal results in all regards from a sweet spot. That's why any full range stereo speaker has woofers. 4 even maybe if you want the right bass throughout the room, not just the sweet spot.

I have those same Pioneers matched with my Cain & Cain Abbys. My
room is 17x35 with cathedral ceilings. Not huge bass, but fast and musical.

A properly set up T5 will work. I suggest that you use the speaker outputs to the REL rather than going through any other crossovers. Proper set up ( location, phase, volume, crossover setting) ) is crucial and differs from other subwoofers. I would suggest you set the crossover point at 55
Herts to start with. Don't be afraid to place the sub in a corner, facing across the room. My son uses a T5 with very good results

I agree with Mapman and Bob Reynolds. And also remember when I was thinking of adding a first sub to my stereo and had no measuring capabilities - or understanding, for that matter. So can relate to the OP maybe getting overwhelmed.

I started adding a REL Storm III and loved it. Paired with B&W 804S. The speakers were used full-range, just like REL recommends, and the sub augmented the speakers low frequency output.

Then I got into acoustics, got myself stuff for measuring, experimented, etc. Eventually I bought two Rythmik 12" kits and built very heavy sealed subwoofers. Rythmiks are outstanding subwoofers, and much cheaper than REL.

Initially I set up the Rythmiks to augment the low end and measured and tweaked a lot. My amp is tubed, so I then experimented running from the preamp (also tubed) to each SW crossover, and feeding my main amp everything above 80Hz. To my surprise and despite the lower quality of the parts inside the included crossover, I liked the sound better. I think both the tube amp and the 6" woofers are happy not attemting to reproduce below 80Hz.

I definitely recommend two smaller subwoofers over a larger single one. And by adding even more subwoofers you can make the low frequencies response smoother across the room. See Earl Geddes papers on this.

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.

Every REL sub I have heard has sounded very nice. A Revel b15 is a heck of a sub AND with its built-in EQ should be able to integrate well.

REmains8-

REL makes the best subwoofer. Model t-5 should integrate nicely into your system. Keep me posted & Happy Listening!

The "precise" volume setting changes when needed...I instantly know when it's too hot or not, but I have a "default" setting for most of the time, and stuck a "chicken head" knob on the volume so it's easier to steer.

Agree with Bdp24 - Rythmik f12g. Can easily fine tune to fit any room.

JL Audio now makes small, lower cost sealed 8" and 10" subwoofers in their new Dominion line. Based on my listening to other JLs, these should have no trouble blending with the mains and adding body and dimension to your setup.

Does anybody else use something like the "high level" speaker output driven input like REL?

Velodyne. I had a Velodyne SPL 1000 about 10 years ago with speaker level inputs. The SPL series was replaced with the optimum series.

Don't rule out the Legacy Audio Subs. They make 4 models: the Goliath, (The ultra-high performance Goliath XD subwoofer from Legacy will match the dynamic range of even the most efficient speakers. Massive motors on dual 15" Silver/Graphite/Rohacell woofers provide world class performance and will handily deliver 130 dB of low distortion output. Bass pressure in rooms will rise and fall so quickly that room boom and droning is not an issue. Kettle drums will roll and thunder, kick drums will have visceral punch and pedal notes will surge with this Goliatath of a subwoofer.)
The LF Extreme (
The Xtreme XD has the power and speed to keep up with Whisper. Dynamic braking, a Rohacell/Silver/Graphite cone and additional ICEpower® amplification make this unit absolutely world class. Bass with BOUNCE, not bloaf.), the Point One HD, (The Point One subwoofer is one of very few subwoofers that can actually enhance the performance of the Focus HD speaker systems. Providing total control over level, phase, and crossover frequency, the Point One also boasts Legacy’s unique BLEND feature. Now with balanced inputs.), and the Metro, (As a 16" cube, Metro is the smallest of our renown subwoofers. Capable of an amazing 117dB of output, Metro is designed to augment systems with woofers 10" and smaller.)
I use two LF Extremes in my system. They blend in well with the Focus SE, the Helix, and I'd hope the V as well.