As someone that went through the somewhat expensive trials and tribulations of trying to get the best out of the HD650 years ago on the cheap I just have a very hard time seeing the Magni 3 as the least competent at driving them out of everything he tried, most of which I too have tried.Not sure about this comment, how is the person intent on holding back the HD650? To me, he/she is trying to get the most out the HD650 since they went out and got the Magni3 thinking that it would be an improvement over their current gear.
I will admit I do not yet have my Magni 3, but over the years I have put my HD650 through the paces on the ASUS Xonar DGX, O2+ODAC, Magni 2 and the Asgard 2 and there is just no way the Realtek ALC1150 is driving them better than the Magni 3. I spent close to $300 buying the best motherboard I could get with it and on paper it sounds real great and being able to even push even 600 ohm headphones but I genuinely feel even the ASUS Xonar DGX edged it and it pales in comparison to the O2+ODAC, Magni 2 and especially the Asgard 2 in being able to drive the HD650 to insane volumes.
That said, I personally think I got the best out of my HD650 or at least finally realized its full potential after getting a somewhat decent DAC in the O2+ODAC combo which by many accounts is not even the best in the sub $200 category. And reading 16 Bit Bowser posts I get the feeling that might be the issue. So if he is using his Magni 3 just to amp a terrible source like an old motherboard then yeah, I could see his iPhone 5S and even CD player with an amp from the 90's sounding better as 90% of on-board audio is complete crap even when amped by a great amp.
Worse thing I ever done to my headphones was neglect getting a decent DAC after buying into the notion they do not make much of difference. They do.