Sounds WAY better on the WA6 compared to the NA7004. I wouldn't think much of the NA7004 headphone section anyway. To give you an idea for reference: The HD650 on the WA6, which I thought was a terrific match so far, sounds muffled, I have to go higher on the pot, soundstage is smaller, less detail, less clear, less separation, less extension. Man, I tell you, these X's were worth the money

VERY happy on the WA6. I'm sure they sound even better on better amps from what I read; I just don't have the luxury to go there (yet). As for power, the WA6 is not a particularly powerful amp (at least compared to the likes of the Mjolnir and such SS's). I usually go to 11-12+ with the HD650 on the WA6. With the X's, it's unbearable at 12, and I can listen quite loud
I tried the WA6 on the higher imp. and lower imp. setting. I like the higher imp. setting, because it's just more open. I switched back and forth a few times to see if it was just a louder sound that gave me the impression. But I don't believe so. The vocals are just more transparent. I don't know why. The settings on the amp say "1-99" for the low and the X's are certainly in this range. However, I like the 100-600 setting. I liked the 1-99 for the HD650's. Supposedly, they're 300 Ohm. I'm starting to think Jack has the switch reversed. I had the hitch when using the HD650 as well, but the X's confirm it.
Anyway, there you have it as far as the WA6 goes, quick and dirty. I listened to Diana Krall Temptation, 40th Aniv. Linn Records in ALAC, and some ALAC Mozart Sonatas. All great. I love these headphones!
I think the Mjolnir is coming in today, so I'll be posting on it over the weekend. I'm listening on the Central Station now to get a taste of SS sound. It's not as melodic or sweet as in the WA6, but as far as power goes, no problems. there's a pick bass in the beginning of Temptation for example. I can hear the fingers on striking the chords much better on the WA6 and I have to look around the room every now and then to see if someone just walked in with an upright bass. The snares are still very quick and clear. Definitely not a dark sound overall. Very live. That's all on the WA6. On the Central Station, not so warm sound. More crisp and not as detailed. Again, as far as power though, no problem. I have their stepped resistor at 10ish and it's pretty loud. Same as with the HD650 pretty much.
One thing, though: They ARE heavy! I've been used to the HD650

Very worth the headache though - ok, not that bad, but pretty heavy. I'll probably get used to it.
Hope this helps for now...