<font color='#000000'>dmoss,
That ATI you picked out would be great! You can never really have too much clean power. At that discounted price, you can't go wrong. I to have had great dealings with Onecall.
In regards to the bridging concern, I would not recommend using one or even two of the channels in a bridged fashion to push a passive sub. There are a number of problems you will run into with this configuration. I would possibly use the multiple powered channels to implement "bi-amping" if my speakers had multiple input terminals. For example, you could implement four of the powered channels on the amp to drive just your left and right main speakers, while only using the remaining channel to push just the center. The only benefit I could see to that particular setup would be for 2-channel audio bliss, only if it were called for.
If you do decide to bridge channels to feed a single set of input terminals on a speaker, just be sure you don't run into any impedence issues as to not damage anything. However, I don't really see the need to bridge such a powerful amp unless you had extremely inefficient speakers, listen at concert hall levels, and had to fill a huge room with sound. I think the the 250watts of each channel could independently control the axiom speakers with ease and finesse.
I would probably recommend just staying with the basic 5-channel (amp) setup mode, and enjoy.
best,</font>