Like zod, I have the Senn HD555 (comfortable) as well and really prefer the cans for late night movies rather than using night mode, etc.

The fact that a headphone output was added to the feature list for the 990 was one of the major reasons for me upgrading from the previous 950.

As far as auditioning headphones, I read up as best I could and then blindly pulled the trigger based on rep and reviews. FWIW, the Grado 60's also sound very nice and are an easy push for the 990, more emphasis on the mid-bass though.