How much amp do the Sennheiser HD650 (HD6XX) need?

In this video I talk about exactly what it means for a pair of headphones to be amped properly, and go over several options from a wide variety of price points. SMSL SAP 7 (not recommended): http://amzn.to/2lGLON0 Fiio E10K: http://amzn.to/2mw9xP4 Massdrop O2 (when it's active): https://www.massdrop.com/buy/massdrop-o2-amplifier?guest=true O2 Amps on Amazon: http://amzn.to/2kWL7SD Schiit Jot and Fulla 2: http://schiit.com/products HD 650: http://amzn.to/2moC4H8 Amazon: http://amzn.to/2lSUFOr