In a review on the X600.1 it did (can't remember the exact numbers) roughly 617 x 1 @ 4 ohms, 712 x 1 @ 2 ohms, 1 ohm was not tested. So even though it's supposed to be 600 wrms from 1-4 ohms it actually increased output as the impedance lowered.
The RSD might not be as "over built" as the XENON, but there's nothing wrong w/either. Go w/what fits your power requirements best.