Your ported box is pretty small....

Anyway, 600 watts a piece should be fine. Many have been more than satisfied with a JL 500/1 on their MAX, so running 2 off of a DEI should be OK. As for volts, I'm not completely sure, but would assume I'm always above 12V with the car running. I used to have a digital read out cap in-line with a Memphis ST1000d powering my S15L7 and the lowest it ever dropped with full volume was around 11.8-11.9V, and that was at a very hard hitting bass note. I rarely go that loud anymore in my Si.

If in the future you want more power, sell the DEI for $300 and pick up a couple JBL BP1200.1's, so that each sub gets its own 1200 watts!!