The other option would be to go for a multi channel amp such as this,
This would give you 500w at 4 ohms and 4 outputs (although at over twice the power of you mains it may be a bit overkill.
Not sure that I understand what you're saying here - and apologies if I've misunderstood - but the standard "rule of thumb" for otput power for amps is 2/2.5 times the rated RMS power of the speakers. That would make the amp you mention almost right in the target zone...
Again, apologies if I've misunderstood your point.
... O/P have you given any thought about crossovers/filtering to help the speakers by giving the subs only the bass to deal with and the tops a 'bass-minus' signal?