Did you consider replacement, I have the same headphone like you (HE6sev2 with 4.4mm balanced cable), S17 Pro temperature 55-56 degrees at 100mA and 10 degrees cooler at 50mA. Ambient/room temperature of 20 degrees. DAC is Gustard R26 connected via XLR balanced.
I don’t have temperature issues with any of my other headphones (regardless of impedance, sensitivity, balanced or SE): HE400i, Ollo S4X, Beyerdynamic T90, HD650, ATH-r70x, Philips X2, ATH-a990z, Fostex T20rp, K712 Pro.
Just love the sound, especially with the HE6SEv2 !