If you want to drive the tweeters seperate from the 'speakers' then you're going to have to go with multiple amps. I've never seen an 8 channel amp and if I did, I'd have to think its total crap.
Like the suggestions above, if you use a passive crossover (which is usually included with the tweeters) you could run 2 of your speakers and the 2 tweeters off of 2 channels. Then you'd only need a 6 channel amp. There are many amps marketed for exactly this purpose... 5 full range and 1 subwoofer channel.
For example:
http://www.cardomain.com/item/ABNA5100T