You only worry about output impedance if you have headphones or IEMs that are low impedance because the ideal ratio is ten to one, with the amp output impedance being ten times less than the headphones impedance.
So the sound is better.
OTL amps (output transformer less) naturally have a higher output impedance, so they sound better with headphones of higher impedance.
They may still sound OK, but low impedance headphones and IEMs can pick up him and noise floor more easily, so you would need better amp.
Transistor/op amp based amps don't have these problems as they can have very low impedance.
So my suggestion would be to go for a hybrid tube amp which uses solid state for output,
OR
If you can afford the best, a tube with transformer out is probably best if you like tube sound...
The only drawback with these designs are the expense of the output transformers and the lower power output which some cans like the Hifiman HE6 need a lot..
So to sum up.
This amp is nice for a higher impedance cans in the range that the specs actually state, in its power output Specs they state the usable impedance and that would be the ideal range.
