How to amplify 4 Main/Front Speakers

If you have the GFS-6, then there won't be a problem. Just switch back and forth between whatever speakers you want to listen to.

I unhooked the 8 ohm 303s from the amp and am just listening to the 640s and WOW do they sound better. I have now become a stereophile, formally, officially. The crazy thins is that I've had the 640s since about 1994 and never fully appreciated the sound they produce. From the things I've learned on this forum and elsewhere about making sure they're positioned correctly and far enough from the wall, I've re-discovered my own speakers. Thanks, all!

I use an Adcom GFS-3 on my second system. I usually select either my main, or deck speakers. You can play multiples at once. I do this from time to time when my house and yard are full of guests. Works fine.