I'll suggest using a decent set of monitors (speakers) in addition to your headphones. If you have a stereo, you can start by patching your computer sound output throuth that. Set it up correctly with the speakers forming a triangle with the listening position, relatively close to it, like no more than about 5 feet away.
Listen to some similar well recorded music to get a good mental idea of what good sounds like with that system before working on your stuff as a general basis of comparison. Switch back and listen to that ocassionally to refresh the comparison.
Headphones are great for hearing, identifying and adressing specific details and problems. Speakers are better for checking and correcting the overall balance of things- the frequency balance, the imaging balance, the reverberant balance. Get it sounding good on both. The comparison thing works well with both.
An outboard DAC is nice, but in my experience not nearly as important as the speakers or headphones themselves, along with a good basis for comparison.