My Sennheiser HD800 headphones have the most improvement compared to the headphone amp in the DAC3 DX. Very easy to hear more dynamic slam at high volumes. Before for some quiet tracks, one could turn the DAC3 all the way up when using the HD800's, not anymore! Low volumes definitely sound like they have more dynamics.
The Focal Utopia headphones are a little more sensitive, so the difference from the added headroom is not as great. The low level details are definitely easier to pick out, but this type of comparison is hard without knowing the voltage levels of the DAC3 and HPA4 and having them set the same.
Definitely there is no degradation with the HPA4 in between the DAC3 and AHB2 (and that's really saying something). I can turn the unit down to -89dB and still understand the spoken words if I put my ear into my horns. Too bad these speakers have passive crossovers in them unlike my other setup
But really, for having passive crossovers in the mix, the driver control is very very impressive and distortion is incredibly low. It definitely sounds like a much much more expensive setup (even though retail on the speakers and Benchmark gear is pushing $12k.
The volume control is very neat and works smoothly. The relays sound asolutely cool when you turning the volume up and down too. The unit is very nice. It's a pity I only plan to use one of the analog inputs (for the DAC3 DX input), but I am 100% digital.