Which subwoofer?

Hsu Research, 18” Velodyne, or the “El Pipo” DIY sub ( this would be my choice”. 

@bdp24 

You got me interested! :-)

Which GR Research Kit do you use? The one with 1, 2 or 3x12" drivers per amp? How many in your room? How far away from the walls do you have them in your room?

I have a couple of Rythmik F12 in DIY sealed heavy boxes, with the same plate amp GR uses (A370PEQ), although with the aluminum driver. I've gone deep into DIY speakers and I'm running open baffles above the subs (65 Hz). I was thinking of eventually adding another pair of F12 to create a DBA, but maybe I get a couple of these plate amps and more drivers and have open baffle DBA...

Cool @lewinskih01! I chose to go with two woofers per side, in the less-common W-frame (most people go with the H-frame, including the Canadian woodworker who offers GR Research customers a great frame in flat pack form), which Siegfried Linkwitz also chose for the frame of his OB loudspeakers. For that reason I got the 8 ohm version of the GR Research woofer, rather than the 16 ohm.

Why? The Rythmik A370 plate amp puts out maximum power into a 4 ohm load (as do of course almost all solid state amps), but doesn’t like to see an impedance lower than that (few do!). A pair of the 8 ohm woofers are fine (in fact optimal), but three of them present too low an impedance to the amp. For that reason, if using three woofers per amp you MUST use the 16 ohm version. However, two of the 16 ohm---which produce a combined impedance of 8 ohms---doesn’t draw all the power out of the amp of which it is capable. Wasteful, as well as lower in maximum SPL capability.

I have the OB subs 5’ from the wall behind them, which makes sense since my loudspeakers---also dipoles---are the same. I have the two side-by-side, but that’s because I use 180 as the x/o frequency between sub and loudspeaker (unlike a normal sub, the OB plays up to 300Hz. It can therefore be used as a woofer with any loudspeaker whose midrange driver reaches down to 300.). One thing to know about the OB Sub: as it produces a null to either side of the frame, it MUST face the listening position if you want full impact at that location.

Though Rythmik’s Brian Ding was involved in the development of the OB sub (including creating the Dipole Cancellation Compensation Network---6dB an octave below 100Hz---he installs in A370 plate amps destined for OB usage), it is more Danny Richie’s baby. Brian found the OB sub to sound more "lean" than he cares for, without enough "weight". It is that weight that many dipole speaker enthusiasts/owners object to in "normal" subs. Each to his own!

In a small room just start with one and why not look at the ML subwoofers? 
Decent fair price and it is a small room.

Kenrus, it is a double edge sword. You will need two subwoofers of at least 12". Yes, I did see the size of your room. If you are going to spend money don't bunt. You need two 12"s or four 10"s or you are just adding a woofer on top of woofers. Getting them to sound right under the MLs will be a challenge from an analog perspective, relatively easy from a digital perspective. Digital bass management IMHO is the best method of integrating subwoofers.