This is my first post on head-fi.org, which I think is a great site! I recently decided to get a pair of headphones and a headphone amp based on all the reviews of the HD800; unfortunately, I couldn't afford the HD800 right away, so I began more modestly with the HD650. I also selected the Lehmann BCL headphone amp after reading a number of favorable reviews on this site as well as the fact that the HD800 was initially paired/unveiled with it. I also ordered a Cardas replacement cable, primarily because I needed a much greater length than the stock cable.
Although the HD650 has some positive characteristics, I never came to appreciate their sonics. Even post-burn in, the top end was too rolled off and didn't sound nearly as good as my speakers. One of the things I like the best about my speakers (Avalon) and upstream electronics (Ayre) is their collective ability to make every recording sound very different. With the HD650, the recordings were homogenized, which I attributed to the rolled off treble (even though the rest of the range was very lush and attractive). That, coupled with the uncomfortable grip, caused me to get rid of them.
I recently bought the HD800 (which is still burning in) and had the Cardas cable re-terminated. The sound signature is much closer to the Avalon's, which are known for their flat response and uncolored timbre. I simply love them! It is amazing the difference between the HD650 and HD800. The best advice I can give is to audition plenty of phones and determine which phones work best for you.
