Respecting what you hear, I have to ask, have you tried the 6 on a speaker amp?
I love the 4, but it does nothing as good as the 6 paired with the J2, the bass is the best I have heard and the air and sound stage are only matched by maybe the HD800.
I was actually a bit disappointed when I picked up the 6, I tried many combos including vintage, the Lyr, Emotiva mini and etc. I then got lucky finding a Firstwatt amp at a decent price and it sounds better then I thought a headphone can sound. All IMHO(of course).
Not a speaker amp zealot, just a guy saying what he hears :).
Nope, haven't had the privilege of trying a speaker amp yet, but I will when I can. I had the Emotiva mini with my HE-4 at one point and while it had more bass it was pretty terribly grainy across the spectrum. I'm sure a good speaker amp would be much better.
I can easily see someone preferring the HE-4 over the HE-6, they're just very different and both excel in different ways.
How much did you manage to grab the Firstwatt for? They seem pretty favored amongst HE-6 users. Have you tried any of your other headphones (HD800, HD600, etc.) with the Firstwatt?