I've been researching headphones for months now with a budget of $1,500. I am lucky enough to have a high-end audio store down the road from my house and I have demoed Sennheisers, Denons, AKG, and Grados with multiple sources and amps. Specifically, I have used a Sonos Connect (16-bit, 44.1KHz music) and Cambridge Audio Sonata streamer (24-bit, 96KHz music). For the amp, I was using a mobile Grado amp, a high-end Bryston amp, and a Music Fidelity amp, occasionally using a Micromega MyDAC and a Peachtree iDAC in the chain when necessary. My musical tastes favor rock, electronic, metal, folk, and hip hop. I rarely listen to classical and jazz.
I won't get too into the sources, DACs, and amps, but I can report one simple fact: The higher fidelity the source music and equipment, the greater the distinction will be between two different pairs of headphones. With 24-bit music played through a good DAC and amp, headphones really unveil their strengths and weaknesses.
Anyway, I quickly found that I was a biggest fan of the Sennheiser lineup. Their frequency response seemed to be more neutral, without lacking bass compared with the others. Most of the AKG sets I wore seemed to lack bass, while Grados seemed very harsh in treble and scooped in the mids. Senns just sounded more mellow and musical to me generally. So I narrowed my choices to the HD650 and HD800 (the store doesn't stock HD600s).
HD650s vs. HD800s
Now, I immediately loved both of these headphones, but each had qualities lacking and strong suits when A/B'd back and forth. After listening to the HD800s, the HD650s seemed negatively veiled in treble and lacking general detail present in the HD800s. Conversely, the HD800s seemed harsh and clinical after the HD650s. On certain amps, the HD800s were almost painfully forward especially when listening to rock music like Nine Inch Nails of Metallica. Again, I was demoing mainly rock, electronic, indie, and hip hop music. With classical tracks, however, the HD800s really shine. Strings and woodwinds just sound amazingly accurate, detailed, and live.
Now, after going into the store a couple times, I learned of the new and old version of the HD650s. A few years ago, Sennheiser covertly modified the design of the HD650 and other headphones in their lineup, which lifted the veil in the higher frequencies and dampened the bass a little. I realized the the HD650s I had been demoing were the old version. Also, the HD800s were S/N ~11,000, which is before Sennheiser supposedly made a slight modification to dampen a certain high frequency spike present in the early production up to about S/N 14,000. So, I had been listening to the most "opposite" versions of these two headphones. I managed to convince them to let me open up a newer HD650, which I confirmed is the new version based on the appearance of the driver (more detail on this can be found in many archived threads on this site). The difference between the new and old HD650 is astonishing. It's not just a matter of breaking them in. The treble and mids are much less rolled off, and the bass is much more subdued. Also, there is quite a bit of detail and definition present with the new HD650s compared with the old. The difference between the new HD650s and the HD800s became much more slight. Delving deeper, the HD800s seem to have a wider soundstage where each independent instrument and frequency seem to have more separation, but not just in detail. It's like every sound seems more isolated in the mix, more compressed as well. Music on the HD650s seems much more cohesive, without sacrificing much detail at all. The new version is still warmer/darker, but not by much.
In the end, I went with the HD650.
I honestly appreciated their sound more than the HD800s. They weren't quite as clinical and detailed in the higher frequencies, but they seemed much less fatiguing and more musical. The HD800s are simply harsh with most rock and electronic music. Also, I managed to find more detail in bass with the HD650s, actually. Comfort-wise, they are both extremely comfortable and well constructed. The HD800s feel more expensive in my hands, but feel cheaper on my head. They have a much looser and sensitive fit with wider ear holes than the HD650s.
I can see why people who listen primarily to jazz and classical music would pick the HD800s over the HD650s, but for my tastes, I would have picked the HD650s even if the prices were inverted.
I hope this helps someone out there with similar music tastes to me who is in the same boat trying to decide between these two headphones.
Edited by SnailsAndSlugs - 1/22/13 at 2:13pm