I should have said above "two MORE 555 IIs" instead of "two 555 IIs" so I would have a total of three 555 IIs driving the 5.1 system.
I would say that if you want to stick to Adcom, go with the 5503. If you listen to a lot of two channel and decide you like the 555 II better, you can always set it up to use the 5503 for the center and rears. I don't know much about your speakers, but do you really need that much power for your rears? Maybe if you listen to a lot of music in 5.1, but if this is just for HT, there are a lot of 100 watt x3 amps out there that are much cheaper than the Adcom. Maybe a Rotel RB 976 would be a good amp. It is 6 channels but can be bridged to be around 100 watts x3. The Rotel would be half the price of the Adcom.