Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

You would need 2 AHB2 amps, one for each speaker. They can be run in bridged mono to 4 ohm speakers. There is an application note page on Benchmarks site where they did this with PCM IB2S monitors. 

@geek, have you considered emailing Benchmark and asking them? I have done that with other companies. I admit that chart isn't easy to read maybe they can send you a better one. 

In the application notes on calculating the performance of an amplifer/speaker combination under,  ' AHB2 THD+N and Power into 4 OHM BRIDGED MONO' it shows THD+N is .00083% at 518 W. It reaches 1% at about 590 W. 

According to Benchmark the amp in bridged mono  the maximum unclipped output voltage is 45.5 Vrms (64.4V peak) into a 4 Ohm load. 

That 18 amp number was a mistake if you read further down it was clarified to be 29 amps. I am not an expert just going by what I read on benchmarks site and various reviews. Benchmark used the amp at a 2017 show to drive a pair of 4 Ohm PCM monitors in bridged mode. Reviewers have used it to drive 8 Ohm, 6 Ohm and 4 Ohm speakers in bridged mode apparently with good results. Actually I agree with you and George I don't really see why 1 amp in stereo would'nt be enough with the OP's efficent speakers that have powered subs so this amp is just driving the mids and highs I assume. But he seems to want 2 amps to try in bridged mode and from everything I have read on this amp it can do it. 

Good luck geek, let us know how they work out.