Made by pierce audio, I'll have one of their subs either next week or the week after. Can't wait
From my own experience with the Emp E-41B's, I'd want something that I could cross somewhere between 80-120 Hz. Obviously this will differ with your uses and how loud you'll be playing, but for a bedroom two channel system you'll probably be pushing moderate volumes. With this in mind, having a well designed sub that you can cross at 100-120 and not have it sound boomy or at odds would be at a premium IMO.
There are a ton of mid priced subs that can reach low and sound great when played in the subwoofer wheelhouse, but because the Emp's use such a small mid woofer, a subwoofer that can play 80-140ish comfortably is really what I'd want with mine.