Thanks so much for the additional info about the Benchmark amp in your system and room. I think my room is a little smaller than yours, which could affect the amp-speaker match a bit. My current amps are the PSA M700 monoblocks, which have plenty of power and sound really good, too. Your S300 is a lower powered stereo sibling of the M700, and I wondered how the CS3.5 works with the S300 compared to the Benchmark amp.
I may not be remembering this correctly and it may not apply to the Benchmark amp anyway, but I thought that most stereo amps in bridge mode didn't handle lower impedance loads as well as the stereo version did (which could make it a less good match with the CS2.2). It also seems like the use of EQ down to 20 Hz with the CS3.5 could require quite a bit of juice from an amp, and the non-EQ'd CS2.2 wouldn't require as much power from any amp.
It would be fun to compare my M700s to the AHB-2 directly, but I remain more fascinated with how the new Atma-Sphere Class D amp might sound with my CS2.2s. The A-S amp uses GaN-FETs and is rated at 200 w/channel at 4 ohms. I won't be attending AXPONA next week, but I'll be looking for comments on the sound of the A-S amp at the show before I decide whether to try to demo any of these options.
Thanks again for all your input!