Greyhound,
The AVA Operating Instructions that came with my 550 state "The amplifier will drive any rational speaker load down to and including two ohms of any design type as long as the five ampere quick blow output fuse rating is not exceeded. These speaker fuses are adjacent to each output and will pass a full power (280 watt/ch) music signal into an 8 ohm load..."
If Frank says the power doubles at 4 ohms, it does.
Lastly, Frank states that using an AVA Bridge theoretically quadruples the power of each amp. In this instance, a 550 would pass more than 1.1 kW per channel!

Frank also has a pretty fine, customer satisfaction based, satisfaction guarantee. Give it a shot before the impending price increase; you'll be more than glad you did.
Have fun,
Jerry