I didn't say that there are no difference between the cards. All I said that the difference was too small for me to make the upgrade worth while for me.
I hear you.
As I said, I can understand that there would be
little difference; just like you saying you're finding it
difficult to tell the difference. I was merely emphasising that the difference is there, because your post might have come across differently to a newbie (affectionate term for someone reading from the sidelines) due to your choice of words which could be misconstrued, but by no means are incorrect.
And with your main point, I do agree that most of the time, assuming the source(s) are as near-perfect as possible (which you have yet to demonstrate experience with, which is important), the upgrade over a cheaper amplified setup (i.e. U3) is small and quite a much of a muchness, especially when listening to lossy sources...
The HD555 don't really need amplification, and the same would apply to all the other < 64 Ohm headphones like the HD595/598.
From Sennheiser it is only the HD600/650/800 that has > 64 Ohm nominal impedance.
I hear exactly what you're saying.

Again, insofar as saying they
don't really need amplification, I have to point out again that, since we are or at least I am trying to be as technical as possible, your choice of words might lead a newbie to misconstrue what you're essentially saying. I am purely raising these points for the benefit of them, and not entirely us.
I know very well that fundamentally we both agree that amplification does benefit headphones such as the HD555/558/595/598, even though they use a "relatively" low impedance. I am very sure that we both agree that most, if not all headphones (perhaps even buds?) benefit from amplification.
This leads me to saying that I would never recommend someone buying headphones such as those mentioned, unless they plan on getting some sort of amplification down the line. My point is, your wording may lead those to believe otherwise, in which case the headphoneswould never reach their full potential, and really shine.
I have the Xonar settings at 2ch, 192kHz sample rate, Headphone out with normal (0dB) gain selected, Hi-Fi for the DSP mode and the EQ is on default. I've tried all the amplification settings, but at Extra High gain, I can hear a hiss when the volume is turned up quite high.
That is the correct setup, so you've no doubt researched it.
To all newbies, Hifi mode is imperative as it adds no post processing to the audio, which would render it different to the way the producer intended.
I'm listening to classic (Sarah Brightmen), pop (Roxette & Katy Perry), club (Basshunter) & rock (Evanescence) music - all of which are 320kbps @ 44.1kHz MP3's. Perhaps I should listen to the music directly from the CD's, or rip them with FLAC
I implore you to, at the very least, compare with FLAC. I'm interested to hear the results. CD's, even better.
I would have to say that our music tastes differ vastly, and that the music I listen to is of a more technical nature. Classical is a good choice, but I would have more layered, instrumental classical in mind. Although the vocal testing is certainly there.
I have to agree with you that the card is most definitely superior to the other cards and that it tricky to analyse the differences.
I absolutely love the 6.3mm headphone sockets - no more adapters for the HD555
It really is an awesome piece of equipment. I find the fact that the OP amps are changeable to be very exciting. Not that I would ever need to change them, I don't think, as I am 100% satisfied.
I have never had an experience quite like my Essence and HD595. HD555 and up are without doubt the best entry audiophile headphones on the market for general listening.
Like the way a lot of our argument revolves, once you are at this kind of level the differences between them and "reasonably priced higher end 'phones" indeed become
difficult to tell.
Perhaps I should ask my uncle to test it, because he has like perfect pitch, and I'm nowhere near that
Please do. Let's see what he has to say in relation to the pro reviews which can only rave about the Essence.