The rule for impedance is generally 8-10x load impedance vs source impedance for minimal frequency response changes. You
can use the 80 ohm DT770s, but the frequency response may be affected slightly, same with the ATH-M50X.
The reason for frequency response changes is that impedance is frequency dependent. any sort of traditional dynamic driver (be that a speaker or headphone) has a
resonant frequency, at which impedance rises dramatically.
Take this impedance plot that Amir did for the Sennheiser HD650. The nominal (labeled) impedance is 300 ohms, but it spikes to nearly 500 ohms at the driver's resonant frequency around 110hz.
Remember our last discussion on this - higher impedance means less current, but more voltage swing. So, an amp with high enough output impedance (remember, high output impedance means it's not good at supplying current) can have its frequency response affected by the impedance curve of the headphones.
View attachment 109881
I'll also tag in
@solderdude here, as he's actually done the comparisons.