Hey Dr.House, what are the headphones in your Avatar? They look very much like the Beyerdynamics DT990 that I have.

I have not been able to compare many headphones, but I had an open-backed Sennheiser before my DT990, and at work I have a closed-back Sennheiser. The DT990 is by far my favorite -- I think it sounds awesome.

Philippe, remember:
Open-backed = fresh air for your ears, but less bass, and people around you can hear the music too.
Closed-back = more bass, but sweaty ears, and people around you can't hear much.

My DT990 is open-backed, but it has slats making some people call it semi-closed. The bass is on-par with closed-back headphones, without the sweaty-ears feeling. Very comfortable. Sound does leak out like an open-backed headphone though.
Axiom M80s + QS8s + VP180 <-- Pioneer VSX-1120-K <-- Squeezebox Touch / XBox360 / Oppo BDP-93