@shadyJ can possibly shed some hands on light here.
The answer to the question imo is that it wouldn’t compare. It’s a very good subwoofer indeed. But the monolith in question is also very very good. And with two, you have the added possibility of smoothing the FR a lot more. Now for a single seat that might not matter as much as but I don’t think one arendal vs two 13” monoprice THX is fair.
I haven’t looked at the measurement data for these lately, just going off memory so happy to be updated. Lol