In a studio setting, that DAC becomes even more important than an audiophile setting. The DAC's drivers, specifications, and features are what you want to look at rather than concerning yourself with sound quality. Ideally, a DAC should not "add" its own flavor to the signal. If you get EMI, hiss, or jitter, and it's not mentioned in the specfications, then they're technically flaws or deficiencies (i.e. cheap DAC knock-offs, unshielded parts, roll-offs, etc.). Stay away from companies that don't stand behind their products (I'm looking at you EMU).
As for headphone amps, their main job is to amplify the line signal for harder-to-drive headphones. The rest are features, specifications, and aesthetics.
So can you give me a hard and fast rule for "harder-to-drive" headphones? What impedance is hard to drive? Over 80ohm? 100? Secondly, is there a situation where I only need a DAC but no amp? Neither? Both? Sorry for so many questions, but honestly no one is answering my questions directly