See a review of the HP50 here: http://stereos.about.com/od/accessoriesheadphones/fl/NAD-Viso-HP-50-Headphone-Review.htm
Apparently a similar sound signature to the M4U1 (same designer, as PSB Speakers and NAD are owned by the same company), but with slightly less bass and slightly more treble. Overall fairly neutral though.
Similar things have been mentioned on their thread here in head-fi... I found the M4U1s already bass lite (especially sub-bass), so if the NADs have even less presence, I'm definitely not interested.