In my experience, the AHB2 is incredible. I can fully change my system based on the DAC and Preamp I choose. This amp knows it's job - to amplify the signal it's given, no more and no less.
Right now i'm using a fully restored sansui 9090 as preamp and i'm enjoying music more than I have in years.
I've used it with a very neutral, sterile preamp and the music was just that. I've used it with a tube preamp and the sound was excactly what you'd want from tubes.
It's a must-have.