@DjBonoBobo Thank you for the links! I need to educated myself on the nuances of sound and EQ as all this is still very new to me.
@ADU The Denon are low impedance/high sensitivity so, in theory, the Sony should certainly have no problem driving them. I'm okay with trying a balanced cable with these but seeing as this was supposed to be a setup that is at least somewhat portable, buying an amp to connect to the Sony and the headphones are not something I'm keen on doing.
I haven't tried any planars but I'm not against them. I'd have to sell the Denon's if I'm to get another pair of headphones, however.
I listen exclusively to classical music as well as movie scores/soundtracks (think Lord of the Rings) Most everything is instrumental, no vocals. I'm wanting closed-back headphones to offer some sort of sound isolation, although I'm not expected complete external sound cancellation.