Looking over posts from late last night and today the issue seems to be centered on the definition of tube-y.
Mark's take is .... This headphone longs for a warm, full, thick and heavy, almost stereotypically "tube-y" tube amp.
The stereotype obviously remains tubes are inherently thick, heavy, slow, rolled off etc... etc...
As jamato8 pointed out, a tube amp does not have to sound this way, and IMO, I believe a tube amp should not sound this way.
The SP amps as a breed are not overly warm or full and have a realistic tonality as close to spot on as I have heard. The SP amps do not in any way sound slow, rolled off or lacking in dynamics. In fact, the dynamic shadings of the SP amps put most SS amps to shame. The SP's do have that organic, liquid quality so prized in tube amps. But, this is not a coloration as much as an indefineable character that sounds more like reality. Resolution of the soundstage size and placement compares favorably with any amp period.
Another point, about tubes in general, is some of what makes tubes special is not what they do, but what they do not do. Tube amps do not generally thin out the tonality, add aggressiveness to the upper mids/ lower treble, add grain and grit to instumental textures or portray a faster than reality sound. So, even if tubes did not offer advantages they can be prized for the flaws they typically avoid.
Now, coming full circle back to my original point. The SP amps simply do not have the stereotypical tube sound one would expect based on the long standing tube stereotypes. Mark stated that the sterotypical sound was what the headphone needed. This headphone longs for a warm, full, thick and heavy, almost stereotypically "tube-y" tube amp. I only interjected because what he is asking for would not be what he would get with a SP. I was trying to be helpful not argumentative so this is my last post. I do hope this post helps clarify my stance.