I think this might be a useful discussion. We all talk about our favorite headphones, but because we all hear in different ways we have rather different takes on them. So, let's take headphones out of the picture for a second and see what we're really after in a system. Then, we can put them back in and figure our what kind of a system really suits our needs.
So, talk about what kind of sound does it for you, and what you ideally want to hear. Let's not all say "musical" since different things make the sound musical to us in different ways. Rather, let's break it down into individual components of the sound signature, and see what we like and how it makes things click for us individually.
Here's what I want out of a system:
Naturally, I want the system to disappear and leave nothing but the music behind. But, since that's not really an answer, this is what makes a system disappear for me:
First of all, the tonal balance has to be right. The tonal balance that I prefer is a sound that's fairly flat through the bass (on headphones that implies a very slight midbass emphasis to compensate for a lesser tactile feedback in comparison to speakers - but much less of a midbass emphasis than a lot of popular good headphones have i.e. HD650), flat through the midrange and on to the upper mids, and very slightly recessed in the treble. This, I find, let's the treble sort-of pop out of nowhere, and makes it more airy and delicate, as opposed to a more foreward treble, which puts extra treble where there should be none, and makes for a metallic, edgy sound with many modern recordings.
Secondly, I'm after detail. A whole lot of detail. I realized that I'm a detail freak when I found myself wishing that the SR-404 had more resolution. I want to hear each and every tiny nuance to the sound, and if it's hyperdetailed past the point of realism, I really don't care. However, I want my detail to be presented naturally, without shortened decay and cut-out transients like the Etys do it.
In terms of speed, attack, and decay, I want a system that's extremely fast, has a fast attack, but a long decay. This is very tricky to get right, but this is what tubes do basically, lengthen decay until sound sort of hovers surreally in midair, while keeping a good attack and a good speed overall. I love this sound, and this is must in my system (this is why I don't much care for solid-state anymore).
In terms of soundstage - I prefer a forward sound with soundstage. What that means is that I want to be right on stage with the musicians, but I don't want to lose the dimensionality of the background sounds. I want a forward foreground, but a distant background. The SR-404 does this really well I think, it's a Grado-ish presentation with actual soundstage width.
I want a very saturated tone color. I don't want things to be greyish or metallic sounding, or woolen and murky and overly lush. I want to hear the body and tone of the instrument first and foremost, but I also don't want to lose texture like Sennheisers do sometimes. I want it all!
Here's what breaks a system for me: wooly, loose, undefined bass (HD650), shortened transients and an overly dry sound (ER-4S), insufficient speed to deal with complex material and many layers (HD580/6x0), metallic treble that sounds artificially hi-fi-ish and not refined (A250 and sometimes K340) and an uneven, peaky midrange (SR-404).
Lastly, here's what I'm more tolerant of: frequency roll-offs at either extreme, though I'm less tolerant of bass roll-offs than treble roll-offs. The SR-003 is pretty rolled off at both ends, but it still sounds quite balanced to my ear because what is there is of a very good quality. Ditto for the ES2 - it doesn't have much deep bass next to the E500, and it's treble is not as extended as an electrostat's obviously, but what's there is very, very good.
When all of these factors are in place, I can basically relax and let the music flow. But, quite frankly, a system that meets all of this is going to be outrageously expensive to begin with, so chances are I'm not the only one that's going to relax and let the music flow. I've heard something like this out of the HE90, but I can't of course say for sure, since it's been so long since I've heard one. I've also had very good results out of several systems, specifically the K340 and SR-404 when they're driven by tubes. Those systems had failed at one specific issue or another, but they have come very, very close. The least offensive, so to speak, headphone in my collection is the ES2. It does nearly everything right to my ear, but I need more resolution/refinement/quality in general out of my main rig (though this is not a knock on the ES2 as it's quite amazing for what it is). I've come to realize that I love tubes, and without some glass somewhere in the signal path, I really can't relax and let the music carry me away. The fast attack/long decay/saturated tone color sound (let's say coloration) that tubes add just does it for me in nearly every way.
So... what does it sonically for you?
Once again, I think this will be useful, since we can talk about where we're coming from in a more idealized, metaphorical manner, as opposed to saying what pieces of gear we do and don't like.
P.S. Sorry for the length, but if we're going to talk in-depth about abstract notions of good sound, then we might as well make it long
