preamp matching to poweramp

What does one need to take into consideration when matching an amp and preamp? The models I am currently looking at are the Opera Consonance M500S Signature 300B Monoblocks running at 18w and the Decware taboo running at 6w. I may have found a preamp for the Opera

http://cls.audiogon.com/cgi-bin/cls.pl?preatube&1141938743

but the decware is a bit of a question for me.

I gather you should try to keep with in the brand, but I need more inputs than decware preamp offers and I dont have money for 2 preamps.

Thanks.