Headphones

so if the headphone power calculator

problem with it is that it calculates based on 1000Hz impedance, not on highest impedance peak you should be calculating with, HD800s is rated at 430 Ohm at 1000Hz and it uses that number instead of it's 680 Ohm peak at 93Hz

also some amps low gain modes are done properly, some are just high gain with pre-amp basically, very lazy approach, low gain should be more linear, if your AMP can't give same amount of power(mW) on lowest impedance drop and highest impedance peak it will change the sound, how much it will change it depends on how big the difference is, all the good sennheiser and beyerdynamic stuff are famous for this, HD800s you might realistically be able to overcome impedance peaks with a powerful amp, but some things just won't happen like T1 which peaks at 1400Ohm at one point and it will sound different on every amp

tldr:/ I would only consider that calculator to be 100% correct for planar stuff because they have VERY flat impedance curves, very rarely some $1800+ dynamic driver headphones also have flat impedance curves like Sony MDR-Z1R