The PRO had that super thin film driver didn't it? Like 1 micron or something. Whereas the NB were like 2 micron. IIRC, even HE-1 is thicker than the PRO SIGMA ... (2.4 micron vs 1) . Maybe too thin? I know the thickness/ thinness is not everything but... it's just weird to think about XD
Also, the advertising on the SIGMA can't be beat (pun intended)
The NB are magic, but cannot help but wonder how more "advanced" the l700 driver swaps have taken them...
The x9k + their TOTL amp sounded pretty darn good as well, and I preferred the sound to HE-1. That is not to say the HE-1 is bad. I would say it's the most correct sounding headphone i've heard. The x9k was more romantic to my ears