The HD 800s' open quality certainly increases their sense of space and depth compared with most other headphones. The downside to that is, depending on the recording, the HD 800s' sound may seem too distant or spacious to some buyers. We liked it, though--a lot.
That open quality was definitely a plus for our home theater auditions where the sound almost appeared to come from the screen. We watched the dramatic "State of Play," and the HD 800s put us inside the newsroom with ringing telephones, the clatter of keyboards, and voices of other reporters filling a huge space. We soon forgot that we were wearing headphones and lost ourselves in the film. The HD 800s have a special affinity for movie dialog; it was the most natural we've ever heard from a dynamic (nonelectrostatic) headphone.
Moving to action, the "King Kong" DVD's ample dynamics were very good, but lacking the impact we heard from the Grado PS1000 headphones. During the scene with the rampaging dinosaurs, we could almost feel each thump on the ground with the PS1000s. By comparison, the HD 800s softened the blows.
Up to this point we listened to the headphones exclusively over our Onkyo TX-SR805 receiver. For CDs, we switched over to our Woo Audio WA6 Special Edition headphone amplifier, which significantly improved the HD 800s' overall sound quality, most notably in the dynamics and resolution of fine detail.
With CDs, the HD 800s sounded clear but laid-back compared with the Grado PS1000s and Ultrasone Edition 8 headphones. Those latter two had much brighter treble and more bass oomph than the HD 800s. So it was easier to follow the guitar lines when rocking out with the Rolling Stones' remastered "Sticky Fingers" CD. The bass and drums were more "live" sounding with the Grado PS1000s, and to a lesser extent, the Ultrasone Edition 8 headphones.
Honestly, we think those two are less "accurate" than the HD 800s, and if that's your top priority, go for the HD 800s. But for our tastes, the Grado PS1000s were more exciting and engaging. The Grado would be the one we'd buy.
To finish up we played the HD 800s over an iPod. The sound was spectacular in every way but one: it couldn't play very loud. It was loud enough for us, but if you really like to crank your tunes, the HD 800s won't be a good match with an MP3 player. The Ultrasone and Grado were better in that regard.
The Sennheiser HD 800s are in the top tier of ultra-high-end headphones. If you're in the market for superluxury ($1,000-plus) 'phones, they should definitely be on your list--if not at the very top of it.