I've researched, but I'm still at a bit of a loss. Here is the riddle.
Hifiman he-6 Impedance: 50 Ohm - efficiency 83.5
Grado impedance: 32 Ohm - SPL 98
Sennheiser Impedance: 300 Ohm - SPL 97
Beyer DTxxx Impedance: 250 Ohm version SPL 96
The Hifiman cans are the most difficult to drive.
Is the impedance measured on a different scale for orthos? Efficiency plays a part here, but I still remain confused as to how to use these numbers when picking an amp for any of these.
I'm sure this is a thread that has been up before, but with the BS search on head-fi, I cant find it.