Hi Jonathan thanks for the interesting link to the Head Acoustics HPSI!
I know this company since I used their equipment during my bachelor thesis.
I used the HPS IV power amp (three models later) with the PEQV preamp/
digital eq. Here's a picture of the setup with Monkey Forrest in the background:
As far as I have understood from the manuals the HPS IV is just an poweramp
without volume control. You need the PEQ V for volume control. The HPS IV
is calibrated with/for a one specific Stax headphone by the company itselve.
The calibration is about exact volume match (I saw the certificate) up to a
100th of a dB between the channels. In this case as you can see for a SR-303.
Now they told me that they use 307 and 407 when I was asking what phones
they use now. The price for a PEQ V + HPS IV + 307 combination is about
1) PEQ V 3.193,00 €
2) PVA IV.3 2.730,00 €
3) HA III 659,00 €
The PEQ V is for manipulating the frequency response. There are several
eqs: for diffusefield(ISO), freefield (ISO), for ID (identity curve) flat and user.
There is also a button called filter. That is for a better perception of sound
(I don't know how to better describe). It does this and is called subjective filter:
Each PEQ V is again calibrated for a specific headphone as you can see
on the photo. If you zoom in you can see the matching serialnumbers on
the PEQ V and HPS IV.
Now to the important part: Yes I took my own Stax headphones (007MKI
and Sigma) with me to test it with the Head Acoustics setup.
I must say it's a good amp but all the features are overkill (like calibrated
volume for audibility tests etc.). A SRM-717/727 is better to use I think. First in
usability and second soundwise not really worse and a LOT cheaper. I would
not recommend that amp without a proper preamp due to lack of volume control.
But the most interesting thing about this system is that ability if you need it
for scientific research (like in sound desing of cars etc).
I hope I could help.