Using the iFi iESL, I initially tried one AHB2 amps to power the Stax X9000's, but it wasn't loud enough. I then tried two AHB2 amps and was really pleased with the headroom (very seldom do I see the Benchmark lights clip anymore unless I have things uncomfortably loud).
Now, I finally have another way to power my electrostatic headphones, as I now own some original Stax SR-009's and a BHSE. To my ears, the BHSE is really transparent too.
However, the iFi iESL + two Benchmark AHB2 amps + DAC3 + HPA4 combo is so good, it may even surpass or at the very least be as good as the famous BHSE (or any electrostatic amp for that matter)!?! I wasn't expecting this, but with the Stax X9000, I can drive the BHSE into clipping while the iFi iESL combo pictured below still has around 3dB of headroom left.
With the Stax SR-009 headphones, both electrostatic setups have way more headroom than needed. No way would I turn up either until they clip.
I'm honestly not sure where to go from the iFi iESL + two Benchmark AHB2 amps + DAC3 + HPA4 combo. I doubt anything available is going to be as transparent and powerful. Hopefully one day we see someone with an Audio Precision analyzer setup a test circuit that can measure a setup like this, and compare it to some of the top electrostatic amps.