The HD 800 is certainly considered to be one of the best headphone in the world…. oh, and did I mention there are thousands of head-fiers hating them and their sound-signature?! And using a portable amp on them, well MANY thousands would consider it a sacrilege :rolleyes:
Listening to music via headphones is a subjective thing, we can´t tell you what you are going to like and we certainly can´t tell you what you are going to like „the best“. There are only vague recommendations we can give you…
The best counsel I can give you, is to try to listen to some headphones, before finally buying one. You have to get to know your preferences, your personal taste in sound signature (etc. etc.) before making decisions like this. Before I ultimately went with my PM-3s, I had listened to roughly 20 headphones. Did I make the „right“ decision for me. HELL YES!
Did I make the „best“ decision? I will never know…
Maybe, to give you some practical advice, considering closed cans step-ups from the M50s and to have a wider range of personal tastes accommodated, try the mentioned NADs (they are very good indeed), maybe the Sennheiser Momentum 2 and also the Audio Technica MSR7 (you should be able to buy all three at amazon for under 700$ total I think). Probably, you will hear something you like amongst them.
And btw, if you won´t like the MSR7, the HD 800s probably aren´t for you anyway…