I can't offhand think of any amps that have 6 channels and 24db bandpass on 4 of the channels...

I agree that external crossovers are probably your best bet, and maybe a 6 channel or a 4+2 setup.

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Alpine IVA-W200
Alpine PXA-H701 optical
Blade SE6175
Morel Elate 6 3 way active
Sony XM2000R
MTX 12" 7000 x 3 freeair

Finally moving into the 21st century...