Lexicon's website states the CX-5 is 140w per channel with all channels driven, and is stable with a 2 ohm load. Have you tried using just 2 channels of the CX-5? If Usher's recommended power handling is 150 watts, what is the reason to bi-amp resulting in an increase to 280 watts?
It is best to use an external crossover when you bi-amp, and using different amps is not really a good idea.