It sounded very nice to me. Better clarity and refinement, but still with the ability to really kick.
Whether or not it was under-driven, I really can't say. Relative to a speaker amp, maybe so, but during the few months I spent with them the Ember was all I had at my disposal. If you go that route, I'd recommend a darker tube to help with the treble (I used the 6N6P). This pair also had some aftermarket pads on them (don't know which ones) that supposedly darkened the sound up a bit.
At the end of the day, I could have easily lived with them as my daily headphones, but the extra price just isn't worth it to me. Especially if you decide that you want to really push them to their limit...the extra price of the headphones, plus amp just gets out of control very quickly. The Ember and 500s are such a good match, that I'm content with what I've got.