Not sure about Fidelio L2 but HP50 is as good as TH900 imo.
TH900 vs NAD HP50, in short HP50 has better mids, TH900 better base and well extended bass.
TH900 has a better sound stage but really bothers me with the recessed mids, at this point I prefer HP50.
Only thing that I don't like about HP50 is it gets very uncomfortable after 15 mins listening, feels like someone punch top of my head with a hammer but YMMV.
The problem with HP50 design is that the headband isn't curved, this creates uneven or I should say very awkward pressure in the center of the headband. Very badly design, this reminds me of my old Audeze headphones.
I only had NAD HP50 for 2 days (brand new), I really like the sound signature and quality that I am getting for such low priced headphone, unfortunately comfort is an issue for me,
My HP50 will be in the FOR SALE forum this week.