Actual measurements show the nominal impedance is 440 ohms rising to almost 700 ohms...
General results Sennheiser HD 800s
Channel
Center/Summary
Sensitivity average in band from 100 Hz to 10 kHz, dB/V
105,2
Impedance average in band from 40 Hz to 15 kHz, ohm
442,8
For 94,0 dB SPL need, VRMS
0,276
For 94,0 dB SPL need, mW
0,172
The table shows a most important technical characteristics of Sennheiser HD 800s
*Center channel headphone is received by averaging between left and right channels
Impedance Sennheiser HD 800s
Channel
Left
Right
Impedance average in band from 40 Hz to 15 kHz, ohm
447,7
437,8
Resistance at 500 Hz, ohm
404,0
402,8
Resistance at 1000 Hz, ohm
351,8
351,5
The minimum value in band from 40 Hz to 15 kHz at frequency, Hz
2032,4
2067,7
The minimum value in band from 40 Hz to 15 kHz, ohm
334,1
333,7
The maximum value in band from 40 Hz to 15 kHz at frequency, Hz
71,9
70,7
The maximum value in band from 40 Hz to 15 kHz, ohm
696,3
643,0........................
This would explain why they didn't work well off the tube integrated amplifiers noted above,they are almost always made for low impedance cans...