Do you have other headphones that you've tested on these amps? I could drive my X2HR to uncomfortably loud (not ear-splitting, but probably at least 85+ dB) levels out of a simple, USB-C dongle that outputs 1V. Sounds like something might be wrong with your headphones (or their cable) if they need such high levels of gain to get that volume.

The OP said HD600s, as in plural, not HD600S. I think it's fairly common to use the plural version of a name for headphones.

Oh and for reference, I also have Airpod Pros, 1st gen. The HD600 should sound pretty similar to me, though more accurate. Better bass and treble extension than the Airpods, but not a huge difference. If you find they sound quite a bit different, you should check any sound effects being applied, and also try a different source.

This is good advice here. I don't really get all of the assumption that the HD600 are hard to drive. I've used them on many sources, ranging from a 2022 MacBook Pro (which has high impedance headphone support) to random dongle that came with my Lenovo tablet. They sound fine and approximately the same on all of them, modulo difference in the DAC source itself. Unless I want to do some crazy +10dB EQ, or play it above ~90dB does the source matter (they sound good from the MacBook and obvious clipping on anything else).

I understand not wanting to spend more on headphones and amps, and I am in that same boat. With the HD600 you honestly don't need to.