Proprius As Headphone Amp

As the Proprius has a label on it saying up to 32 ohms can I just check that I won't invalidate my warranty by using them with higher impedence headphones.

Given your positive involvement in this thread I'm guessing that's not a problem.

The nominal impedance of the attached load. (Ohms)

The electrical sensitivity of the attached load (dB SPL per milliwatt)

The dial position of the Proprius (higher positions can produce more noise so choose a position that makes the low-level hiss inaudible)

Proximity of the drivers to the ears (the closer they are, the less power required and the easier it is to hear hiss). This is only relevant with earspeakers and speakers as most headphones have close proximity by design.

Many headphones have removable cables so you can source/make a high quality 4-pin cable if you don't want to use them common ground with the negative terminals joined. But if impedances are high and current flows/oscillations are low, I'm not sure it will matter. I remember the HD800 was clear at 8 o'clock but the sensitivity is too high to use a much higher position without a high noise floor. At about 9 o'clock, noise was creeping in. To use a greater dial range there, a potential divider circuit would have to be implemented to adjust the impedance and tap off voltage.

If you don't mind cutting off the audio jack from the end of the headphone cable, which I believe is where the negative connections will usually join together, they can be separated from each other and connected to separate monoblocks. Not too much of a big deal if the headphone cable is removable and can be replaced with an official spare part.