I would buy the audio interface and then later upgrade to O2 amplifier if I was in your situation. I think the DACs in those interfaces are pretty good these days. Wouldn't drive an SRH1440 off the headphone jacks though.
That is crazy and im jealous.
If I didnt have special real life right now I would probably buy another DT48 and experiment their sound with a couple of different sources. Fun testing. I miss the DT48 "sound".
I tried the HD700 recently too, just out of my mobile phone. Was impressed by the bass, very extended and punchy. Overall very fatigue free sound, lots of detail on the hi-end. I could live with these.