Suggestions for a three channel amp

I'm thinking of upgrading my two NAD 218thx's for a three channel amplifier. (I use the fourth channel for rear center surround) Current system uses a Rotel 1066 preamp with standard Shearwater speakers. Source is the new Sony DVP 999es. I was thinking of the Aragon 8008, but I'm not sure if it will make a difference. What price point do you think I would have to reach in order for me to really hear a difference.
Using a price point as your main criteria may result in your overlooking some good candidates. Among the 3-channel amps that you might consider are:

Lower-priced: Adcom GFA 5503; Rotel RB-993 (there is a used unit for sale right now on A-gon)
Mid-priced: Bryston 5B-ST or 5B-SST; and Bryston 6B-ST or 6B-SST; Marsh HT500-S
Would the Adcom or Rotel really be an upgrade from the NAD? Do you think the BRyston would produce an audible difference given my current system

I have a Bryston 8B ST THX for Sell(it is four Channel amp)
At $2600CDN,box and Manual,if interest,please check my Website at or 905-727-2004 Contact Mike