The full range channels on all of the XD amps are 75w RMS into 4ohms, regardless of the number of them. The sub channel on the 500/3 and 700/5 only plays up to 1khz, it's not full range.
For the little bit extra the 600/6 costs over the 400/4 I would for sure pick it up and enjoy either being all active (technically you will still need something on the tweeter other than the amp crossover) or the extra power from bridging the comps.
I'm currently considering two 600/6 for my front speakers.
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minimum space = minimum bass