Another vote for speakers...
Reference System:
Analog Front End: Shelter 901 + SME V + Orbe SE + ARC PH3 SE
Digital Front End: Mark Levinson #37 + Genesis Digital Lens + Sonic Frontier SFD2 Mk II
Amplification: ARC LS2B MkII + Mark Levinson #333
Speakers: Thiel 3.6
Headphone System:
Digital Front End: Sonic Frontier SFT-1 + Mark Levinson #36
Amplification: Still waiting for the ok2000 HP100-A hybrid amp, and Consonance Cyber 20 from Opera Audio
Headphone: Senn HD600 + Cardas cable
I'm fairly new to this headphone business. My impression so far with headphone listening is that the soundstage is very two-dimensional. With speakers (properly set up, of course), you get a 3-dimensional soundstage presented in front of you. There's a depth and layering of instruments that you can pinpoint from a space right in front of you. You can get a sense of where each instrument is in relationships to one another. With headphones, everything seems to be contained within my head. A Saxophonist would be playing in my right ear, the drummer in my left ear, etc. It's a very strange feeling for me. I noticed that with headphones, I pay more attention to tonality, treble and bass extension, and pace. With speakers, I would pay more attention to the presentation of the soundstage.