Outstanding. Yeah, this line of headphones is simply magical. Recently I read an outline from some site in the Czech Republic on the Sennheiser headphone line, and they coincided that the HD540 was the apex of the Sennheiser line to this day. The open and tonally accurate timbre and musicality are unmatched by the HD580 and later (shifting to a less open sound, adopting the Harman Curve instead of the Diffuse Field loudness curve, like the HD540), whereas the HD800 (which is a return to the same tonal balance of the HD540 generation) suffers from some lack of musicality and integration of the parts into a cohesive whole.
I think the HD540 family extends quite a bit, probably up to the HD565, although I think the HD565 and HD545 are a little in between the HD540 and the HD580 tonally speaking. Below the HD540, it extends as far as I can tell down to the HD520 which I have. Let me tell you, they refer to this headphone as a "lesser member" of the family, but it could fool me. It is extremely similar to the HD540 Ref II, though perhaps it is more a lesser brother of the HD540 Ref I 600 ohm (if the Ref I 600 ohm is that superior to the Ref II).
To offer a counter to your experience and illustrate the magic quality of these headphones, I have been experimenting with the most minimal setup, and I am astonished at the results. It defies logical explanation, even.
The past 3 or 4 days I have been driving the HD520 from an 2005 RCA Lyra mp3 player which has a Sigmatel DAC. I already knew the RCA Lyra is a brilliant piece of equipment, a "sleeper" if you will. The Sigmatel DAC is the same one which was inside the iPod Shuffle 1st Gen, which many people consider the best sound quality in the iPod family up to a few years ago. It is extremely balanced and musical, with all the frequencies in the spectrum clear and present, but it suffers from some steady electronic noise (barely noticeable in most situations, depending on the headphone you connect it to). With my Sony MDR-7506, which are quite sensitive, the issue could be heard when there was complete silence, but it was inaudible otherwise.
I didn't know what to expect when connecting the HD520 to this thing (which to top it all, runs on an AAA battery). But my mind has been blown away. I am getting about 95% of the quality I get from driving the HD520 from my O2 amplifier. Maybe the highs can be slightly more shrill in some situations, but it is a tough call, I would need to A/B. The volume obviously needs to be almost maxed out (though I normally have to reduce it 1 or 2 notches depending on the source material because then it's too loud for me. Also, the electronic noise I referred to (a high pitch) is inaudible with the HD520, probably due to the higher impedance and lower sensitivity, I guess.
I was listening to the Nirvana Unplugged set and the realism is really stunning for this setup. Almost like being front row in the audience. I could hear things with clarity I could never hear with the 7506. Comments from the public, etc. The soundstage with the HD520 is so good that a few times somebody knocked something, or an amp was plugged or unplugged, and I looked behind me because I thought the sound was coming from my door in the living room, then I rewinded to replay and heard it again.
This generation of Sennheisers is golden, just amazing. These are not normal headphones, they are teleporting/time machines.
If somebody had told me that it is possible to drive an HD520 (300 ohm, 94dB sensitivity) with a rechargeable AAA battery for over 6 hours with this quality I would have said they were insane. You have to hear it to believe it.
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