Vocals are definitely more upfront with the L2. While quality and quantity of these two headphones in the lower end is very comparable with a slight advantage for the L2, the L2s excel in the treble region. More life, more sparkle. In the first hours the L2 can get a bit harsh with very bright recordings but that settles with burn in.
My feelings about the NAD are quite similar to yours. When I put them on it always takes some minutes to get used to their signature or, well, them not having any real signature besides being somewhat warmer than neutral. After a few songs they begin to work and I could leave them on for hours. The L2s on the other hand are always catchy from the first note.
I can also say something about the fit: While padding and size of the headband are almost identical, the earcups of the L2 are a bit larger and the ones of HP50 are softer. I have no comfort issues with both.
I've read several people saying that the lower mids are lacking a bit of warmth. Do you think that's the case? How do the lower mids compare to the NAD HP50s lower mids?