Ah so you're using the headphone jack on a speaker amp right? The power output is very low compared to low end headphone amps or even a mobo/apple dongle. Given that the power output increases with impedance, I'm suspicious that the headphone output is coming straight from the tube preamp. This would explain why you're getting better performance on your dynamic driver cans than planars, which are low impedance, low sensitivity (generally don't do well with tubes). It's a bit of a shame because that amp looks pretty sweet otherwise. What I would do if I were you is hook up a dedicated headphone amp to the dac/preamp of the SV-500. You'll still get the tubiness of the preamp and good dac, but yeah you need more juice into Edition XS. They're 18ohm impedance so you're probably getting even less than the 24mW. You don't need anything fancy. Schiit Magni or Atom amp would do the trick. You'll probably notice a performance improvement with your 6xx and Clears as well.