I've owned the 650 for roughly a decade, as such I've had the opportunity to listen to it extensively with the following gear:
Denon AVR1905, AVR2808
Fiio E9, E12a, E11, X3ii, M6
LG V20, V50
Schiit Lyr 1, with various tubes and their SS "tubes"
Drop x THX 789
Astell & Kern Kann Cube
Dragonfly Black 1.2
With almost all of those sources the HD650 gets 'loud enough' but...