Hi I'm trying to choose the best amp
for my setup. I only need five channels
at the moment, so I am looking at
the 7100 or the 755. My speakers are Atlantic
Technology 450 THX (89 db sensitivity, 100-200 watts). Would the 7100 be enough to
power them?

The 7100 would be enough, you will find them to power your speakers for the full range well if your speakers are 8 ohms or less nominal. If your speakers run above 8 ohms, then you will want to go with the 200W amps. For instance, a speaker may be rated for 8 ohms, but in real world testing, it may run nominal at 6 ohms or even 10 depending on the test. Something to look into for yours so you can be sure if they are in need of more or less. Of course, if you don't know, the buying the bigger amp is always a sure thing, ie the 755 and then adding something like the 200's later.

Which amp would sound better? I want my speakers to sound the best, but if the bigger
amp is not going to make a difference in sound then I don't want to waste the money.
I know that a bigger amp will have a bigger
reserve of power when needed, but would my speakers ever need it?

The two will sound similar up until a point in power, then if your volume you listen at goes above this point, then the 100watts will start to intro distortion as you exceed its natural limits. So the question is, how loud do you play them. Couple this with what is the bandwidth that you send to your speakers, if your running the mains below 100Hz, then you really need the extra power.

I like to be able to run at the reference point(75db) or sometimes a little louder for demos
:-). My speakers can only go down to 80hz, so
that is the cross over point. Everything
below goes to a self powered subfire sub.

So I guess in my case the 7100 would be enough. Thanks for your input.