Just for reference, I have the audison 8 channel amp. 2 tweeters, 2 front, 2 rear, and the remaining 2 for sub. Not sure what the cost difference is, but it would take away the decision between passive crossovers or rear speakers.
Another consideration is whether you have enough power for sub at 270W. I'm finding my sub running off the 2 channels on the amp to be ok, but I do have a sub out so I can add a mono amp in the future if I wish to. Your 5 channel Audison amp doesn't have a sub out, so you will have to replace to upgrade.
I'm no audio expert though, so the others can give better advice than me.