I've been using the HD650 as my daily driver for a couple months now and I've found the o2/Magni to power them just fine, you shouldn't need anything more powerful or refined for the HD650s, I couldn't hear a difference between the o2 or Schiit Lyr for example. There are few headphones I feel actually need anything more than something like a magni or o2, and 90% of them are orthodynamic.
I don't think I have any special hearing, but I can definitely hear difference between Magni (haven't heard o2) and Lyr. Not saying that Magni is bad, it's great with HD650, but it's not Lyr like experience for me.
If I would talk bike language I would say Magni and Lyr is like Yamaha R6 a R1 .. R6 is definitely great has quite some hps and it's fun to ride, but it's no R1. Once you ride R1 for some time you don't wanna go back to R6
ps: I'am no zealot, if something doesn't make my music sound better I will just sell it. Won't keep it just for ego thing.