Cold was a bad term. I just meant that comparatively the Gungnir seems more artificial. Its attack to me is overemphasised and its upper registers lacks the smoothness of the NAD. It's less "silky", less natural and "cold" in the sense that I found it less inviting.
Your other points are spot on though. If your description of the Gungnir's warmth stems from its low end presence and slam then I agree. This is actually one aspect of the Gugnir that I miss at times. That's not to say the M51's bass rendition is wanting, just that at times it can be a tad too rounded. The best aspect of the Gungnir's attack and leading edge emphasis is the delineation of bass instruments.
But for me the NAD's transition from the midband to the smoother and extended upper registers distracted less than the hardness and grain I found in the Gungnir's rendition of this region. Could be that my appreciation of the Gungnir's bass and simultaneous issue with its forward upper register gave the impression of a V signature and its mid body was overlooked as a result. Hence the "cold" and "artificial" descriptors.