The engineering of the volume control looks incredible as discussed in their recent application note:
https://benchmarkmedia.com/blogs/application_notes/benchmarks-256-step-relay-controlled-attenuator
I have some Bencmark AHB2 amps in my theater as well as their DAC3 I use with my Focal Utopia's and Sennheiser HD800's, and I am still impressed every time I listen to either system. If the HPA4 is anything like the AHB2 amps I am now accustomed to, I feel like I must pre-order the HPA4 although $3k is a lot of money to me and my wife may not like it as I promised I would quit spending money on gear (and haven't felt the need in my theater after the AHB2s I added in 2017).
For the AHB2 amps (same THX AAA topology used in Benchmark's HPA4 headphone/preamp):
The background sounds in music that were presumably lost to inter-modulation distortion and even my previous amp's much much higher noise floor, are so crisp and clear that things sound absolutely real.
An amp such as the AHB2 has 100's of times lower distortion at low volume levels compared to even the best consumer amps available at any price. I too was skeptical until I heard and owned some, but this lower noise floor, lower THD and IMD of the THX AAA design is very audible once you experience it. Sounds come out of black space with no hiss being heard unless it is in the recording itself. As an example, vocals seem to endlessly decay, and you can honestly hear the room the recording took place.
While a lot of thought and time went into my system such as actively tri-amping 5 channels with FIR filters, room treatments, crossing to multiple subs that are individually time aligned and fed a mono signal, the Benchmark AHB2's were the missing piece I'd been looking for a very long time. Well recorded HD tracks like those from AIX Records are so real now that one is literally transported in time and space.