At the time of wrting the HD800 headphones are currently Sennheiser's top end set of headphones. They claim to have the largest transducers and the most advanced driver technology of any current headphone on the market at this time. These are reference class headphones and a benchmark to compare many other headphones against. Premium parts have gone into the build, the transducer being encased by precision machined stainless steel and the ear-pads are claimed to be hand crafted from a high-quality microfibre fabric.
They are one of the most comfortable pair of headphones I have used and I wear glasses so comfort can be a big issue. I have usability issues with Grado headphones with the L-cush pads on a pair of Grado PS500 and needed to fit the G-cush from the Grado GS1000 to make them usable, although they do sound better this way. The HD800 are as nice to use long term as the older HD650.
They are not cheap with a RRP of £1000 in the UK but it maybe possible to find them a little cheaper in the UK if you shop around or wait for a sale deal. Although they are a high price these are a once in a lifetime buy and set a benchmark. You would need to spend up to 10x the cost on speakers to match these sonically and this is the beauty of headphones. You can reach the very high end of Hi-Fi sound for a price that is at least attainable for the average enthusiast. Another advantage of headphones is the fact that the room has no influence on the sound reproduction which cannot be said of speakers which are highly room and position dependant.
I am going to compare the HD800 to a pair of Grado PS500 fitted with G-Cush pads and a vintage pair of Stax Lambda Pro electrostatics. The Stax and the the Grado PS500 headphones are almost opposites sonically and both excel in one particular area. The PS500 have tremendous clarity, detail, punch and extension were-as the Stax have a much bigger sound stage extending well outside the headphones, but do sound a little darker and maybe pinched in comparison. The PS500 can sometimes be unforgiving on some music due to its slightly brighter more extended sound, were-as the Stax are more relaxed, spacious and less in your face. I prefer the Stax for long term listening since they never get fatiguing and give a better presentation of the sound stage but the PS500 are a much better headphone if you want to use them for mastering and pro audio use to hear the micro details in a mix. The Stax are more of a long term listening Hi-Fi orientated unit.
The Sennheiser HD800 combine the best of these features but do both better. They have a 3D sound-stage exceeding the Stax along with more clarity and detail than the PS500 but without ever getting too much or fatiguing. They have a good balance of detail and extension without sounding forced or trying to initially impress but later sounding too much. The breadth is staggering and they can layer complex sounds with all the micro details laid out. The Stax and PS500 do not have the space for separation that the HD800 has although the Stax Lambda Pro is very close which is not bad for a pair of headphones from 1982. The Grado PS500 are more extended which can initially make the HD800 sound a little dark and dull but the HD800 are just more restrained and better balanced.
The HD800 are equally at home for Hi-Fi use with long term listening and would be ok for pro audio mastering. They are not the most bass rich headphones which I believe the GS1000 do better from my memory of briefly listening to a pair. The HD800 give an exact reproduction of the sound without emphasizing any area. The bass is flat and neutral so if you are a bass freak or want a heavy punchy sound you may find them a bit lean and lacking. However I later found that they get vastly better with higher quality amplification so a good dedicated headphone amplifier is a must. I have read some comments about them sounding bass thin compared to the GS1000 but this is because these Grado's have a richer low end, and again, the amplifier is a major factor here. I would like a pair of GS1000 to compare as I believe they are the nearest HD800 rivals and do some things better. The HD800 could sound a little sterile for some tastes if you are used to a warmer and lusher sound, especially on some amplifiers which the HD800 do not suit at all.
One thing I have noticed about the HD800 particularly compared to the PS500 is that they are much easier to listen to with what initially seems poor or lo-fi style recordings. The HD800 never grate and average recordings are still enjoyable and musical, they seem to stick to that fine line of insight and detail without ever being annoying. My preference in order would be HD800 > Stax Lamda Pro > PS500 with one caveat being that the HD800 are very much more dependant on the amplifier compared to the PS500. I would suggest you audition these alongside the Grado GS1000 and maybe a newer pair of Stax. I would still recommend the PS500 if you can get them for a very good price and prefer raw detail and extension over a more spacial sound. However at the current UK RRP they are approaching HD800 territory and are just out classed. Now only if I had a pair of the new Grado GS1000+ it might be a different story.
These headphones really demand very high quality amplification with good current drive. I found that a Yulong A100 headphone amplifier I used with the PS500 could not push them to full potential and as a result they lacked bottom end extension and grunt. I later bought a Musical Fidelity M1HPA after reading good comments about this amplifier with the HD800 and I can agree that this is a good match and not silly money. It does drive them better, however my older Yamaha AS1000 amplifier is still a better match than the MF dedicated headphone amplifier in terms of richness and bottom end drive which the HD800 seem to benefit from. The HD800 need very careful amplifier matching to sound the best. With the wrong equipment they can sound lean but with the right amplifier these are in the very top tier.