K7 has a lot more power and more modern/better DAC as well. You'd have enough power with the K7 for 99% of headphones you might get in the future as well, it has 10-20x the power of the E10K.

I'd pass on the E10K, if you are looking at the E10K may as well just get a dongle like the Truthear Shio which is much more compact and portable and has both a better DAC and higher power (balanced) for the HD6XX- 4V vs 2.6V.