Personally, I'd just manually calibrate with real sounds such as my voice or fingersnap. Another trick is by playing very quiet at first so when you increase the volume, it will sound significantly louder while actually still low in db.
And don't underestimate other factors. I lost a bit of my hearing due to flu and some medications.
HD600 user here. Had experienced with different dac/amps. I don't think price matters for me, but the kind of tech used for each dac or amps was interesting stuff to explore. The best one for me was the cheap standalone dac from drop and the entry class level A from headamp. Ended up selling those as well because out of curiosity, I plugged my 600 to my sister's macbook and shi..t. I'm a macbook user with no amp whatsoever now
HD600 and ex800st. Macbook. Apple dongle.