Well it honestly depends on what kind of sound you want.
In my personal opinion, AKG's K7XX's sound great with an ODAC +O2 combo unit, however, like the HD650 and HD6XX's, they have great scaling so it's really open to future amp upgrades too.
From Sennheiser, the HD598 is also a great choice. It's a bit 'lighter' in the way it's not bass-heavy, and it's a bit more even, but to some people who aren't used to the sound, it may seem 'lacking in...
