Here's a comparison between Sennheiser HD-800S, HD-6XX, HIFIMAN HE-4XX @ the same voltage level of 0.5V Pink Noise.
The difference is small, balanced Qudelix 5K can drive both Sennheisers to full 4V.
The HE4XX to 3.6 V, 1dB lower, due to it's lower impedance of 43.5 Ohms (values corrected after measurement on real specimen) it needs more current, see here:
https://docs.google.com/spreadsheets/u/0/d/1q_jWA9v-NKCsPc9Rm08au_TlqEsEgUeckeSoaVGfSRM/htmlview
All three can be sufficiently driven with full dynamics to earkilling sound levels.
If you need more level you need to see your audiologist now, or soon
With all three my personal listening level is set way below maximum, even with softer recordings.
Watch out for the fine scale of 1 dB/line, which makes the (small) differences very obvious: