Piggy-backing on what @jbhiller said very well, I use the Cornwall IVs with a pair of REL t7i's, speaker level from a Pass INT-60. The RELs have 8" active, 10" down-firing passive. They're small enough that the OP might be able to accommodate a pair. I've tried to cross them around 30-35, although REL doesn't make it easy to be precise in that regard. As jb said, they're not really necessary, but I like to think they add just a touch to the Cornwalls' palpable sense of presence.
