@earthbound
I have a pair of SVS 3000 Micro. These are very affordable and work very well with the Charneys. My listening room is less than ideal and about 28’x32’ with 14’ ceilings. An old double shotgun that was converted into a single so area has my kitchen, DR and living room. The combo has no difficulties filling my space with sound. My system is wired XLR up to the SET amp (then it is RCA as I understand to put balanced inputs into a SET amp complicates the build and may affect SQ) I use the XLR outputs of the Motherload XL to the amp with XLR to RCA adapter cables and use the RCA outputs in the Motherload XL directly to the subs. When using the XLR out, the RCA remain fully active so the signal goes to both subwoofers and to SET amp. But it took me forever to figure out I could do that. I am sure Aric can give you guidance, as I am not familiar with the intricacies of the custom 300B he designed.
As a side, I also own a pair of Atmasphere Class D monos and rotate them into my system. They are really awesome and sound pretty close to a flea watt amp in my system. Obviously, they have no trouble driving my speakers, but for someone interested in the magical sound of a flea watt system without a sensitive enough speaker system, I would not hesitate to recommend trying the Class D by @atmasphere And they are very reasonably priced even new.