I use Sennheiser HD-580 headphone, and the following summarizes my Initial impression and observation, out of the box:
– Amp sounded very powerful
– Excessive fatique
– Left jack has more gain than the one on left = opposite to what the User Manual stated

Listening experience after 100+ hours of burn-in:
– Amp remain powerful, very dynamic, and enjoyable overall
– It sounds more refined and less fatique than out of the box

Edward K
Ontario, Canada