As with any speaker 2 identical subs would be best, but I have been running the EP400 with my old, very bad sub, and they worked quite well together. I would say that the differences in LFE pushers are quite a bit less than all the other designs. With my set up I really only hear the 400 but my old sub moves the air better so it helps the rumble during HT use.
Having just had the STF3 in my house I know the power the Hsu lines have so I would venture a guess that the EP350 might be lacking a little in that regard, but it would still fill in the room nodes and help equalize the sound. Although Alan's last post about the 350 said it would rival that competitor with 3 capital letters, so it must be quite good.
