Here is what I wrote in the other thread:
"I had that problem with both my HE-400s and my HE-500s. In short, the same answer held true: they sounded thick, slow and muddy out of tube amps (WA6, SinglePower MPX3, Eddie Current Laconic EC-01) and good (for the HE-400) or wonderful (for the HE-500) out of solid state amps. My favorite amp for my HE-500s is my Neco Soundlab PORTABLE amp. Crazy? Sure, but I know what I hear. I've recommended the Neco to two others for use with the HE-500 and they were blown away. Build quality is absolutely phenomenal to match the sound quality and they are relatively cheap.
As to your specific question, the NFB-10SE > M^3 > O2. The Audio-Gd is definitely the best. I thought I heard a bit more noise with the M^3. No particular complaints with the O2, but I suspect it would start to clip if I turned the volume up higher. But to be clear, the sound was NOT muddy out of any of these. The HE-6s sounded marvelous. Check my profile...I know what good gear sounds like. Was this the best the HE-6s can sound? No idea. I'll get around to trying them with my Vincent amp at some point, and we will find out then."
I've spent some more time with the HE-6s and the NFB-10SE since then. The NFB-10SE will put 6W into 50 Ohms. I'm ready to be proven wrong, but I'm confident that the NFB-10SE is driving the HE-6s very, very well. Tons of bass, good spatial imaging. But I don't think the HE-6s will ever catch my HD800s when it comes to imaging, no matter how much power is backing them up.
BTW, I consider the NFB-10SE a real steal at the current Audio-Gd pricing. And the DAC section is outstanding, too.
Regards,
Adam