I have been comparing a HD540II with a HD800 (serial number is above 30000) through my DAC/amp quite extensively over the past few days. Here is what I think so far:
1. Both are equal on tonal accuracy and realistic instrument timbre. Both have exceptional neutrality and are not harsh in the treble at all IMO.
2. HD800 has more bass weight than HD540II so the low frequencies are more audible (better extension). Low end of the HD540II is at a lower level but is still very good.
3. HD800 has a larger stereo image size than the HD540II. Performance feels bigger whilst still feeling natural. HD540II provides a slightly smaller stereo image, IMO.
4. The tiniest of micro details such as low level hiss and background noise are very slightly more audible on the HD800 than the HD540II. You have to turn the volume up further and listen more carefully to hear this low level noise on the HD540II
5. The HD540II has a purer sound than the HD800. The HD800 seems slightly out of focus (sonically blurred in comparison) and I don't know why this is. Dare I say the word "veil"? .... I think that this word sums it up. The HD800 probably requires a very specific (skewed) driving configuration in order to give the same crystal clear purity of the HD540II.
6. Both sound spacious but neither image like speakers. (Even my K1000 does not image exactly like speakers)
7. HD540II has a simpler, more lightweight and comfortable construction than the HD800. Although both are modular.
Point 2,3 and 4 are the result of larger size drivers in the HD800, in my opinion. In some areas, the HD800 is an improvement on the HD540 and in other areas, it is equivalent or inferior. HD800 is physically over-engineered, adding unnecessary weight and expense IMO. It is too expensive for what it provides; I predict that it needs a certain type of amplification in order for the transducers to excel and this should not be the case for a device of this price. A larger driver could be used in the HD540 design to provide the sonic improvements that the HD800 demonstrates are possible.
I would choose the HD540II over the HD800 overall but the HD800 has allowed me to recognize the main weakness of the HD540 design; the smaller driver size... Oh well, at least it's crystal clear... =)