OK had a chance to listen to the NAD VISO P50 today.
Build and comfort wise it looks awesome and seems solid but the cup was smaller than the K545, enough so I was concerned about pain for long listening sessions.
Sound wise I thought it was as good as the K545 but different. It is good enough and different enough I may consider buying it as an alternative to the K545. In my mind they are the 2 best portable headphones I have heard.
Bass - the K545 went deeper but had slightly less volume, the P50 had more mid bass but not in an excess way.
Midrange - The K545 has an emphasis higher up in the midrange, the P50 has an emphasis at a lower point in the midrange. The difference being voices were warmer on the P50 and cymbals had more emphasis on the initial crash while the K545 had more emphasis on the sizzle and decay. I found the P50 a bit harsh on a few songs and was turning them down to compensate.
Treble - I actually found these to quite similar treble responses, liking them both equally.
Sound stage - I think they both have excellent sound stages but the P50 does a a bit better job of creating an out of head sound. The K545 is what I would call a bit more analytical the P50 a bit more organic.
The level of detail between the phones was very similar except that each presented things a bit differently based on my signature descriptions above.
Bottom line sonically. If you have issues with upper mids/lower treble the P50 is a safer option. If you find a bit lower mids to be "shouty" then the K545 might be a better option. If you not sure which one you will like and can't audition them, FLIP A COIN!! because the odds are will like either one
Before I forget. All my testing was done using John's K120 using 24bit FLAC files. The player was stunning ($1400 price it should be!!!) So both headphones were represented in the absolute best possibly scenario source wise.
One final point. The P50 was a much more efficient headphone needing quite a bit less volume as compared to the K545.
I won't be rushing out to buy the P50 but I wouldn't be surprised if I ended up owning a pair some day.