You'll want the Benchmark DAC3 since it outputs +24 dBu from the balanced connection. This means you can use the AHB2 in low gain, which in turn reduces noise and maximizes the performance of the AHB2.
I've used the DAC3 HGC with the AHB2 and it sounds phenomenal. I currently have the HPA4 with a DAC3 B in my system. The HPA4 is a little more convenient because I can easily set the volume between movies and music due to the discrete volume control / display. The DAC3 HGC is now part of a headphone system on my desk.