I got mine last week, and am really enjoying it in balanced mode on HD-650s, driven from USB on my Mac.
The treble, bass, and gain switches all have very obvious effects. Treble and bass of course depends on your source material: the music has to have a lot of energy in the areas where the filters work for it to sound obvious. I'm running at lowest gain, with the volume knob at about 2 o'clock, which is plenty for most music for me. I might turn it up more for classical since the average level is lower.
The impedance switch is pretty subtle, and I'm not sure I can reliably hear a difference on my headphones. But again, I think with high-Z headphones like Sennheisers, the effect won't be very obvious because the impedance of the headphones will be the biggest thing in the circuit. And if the impedance curve is pretty smooth and flat (like the Senn's), then the differences won't be pronounced either. I bet the difference will be more obvious on the HD-800s which have a bigger impedance bump in the bass (it will sound bassier with increased output impedance on the M8), but it will still be pretty subtle since the 800's input Z is even higher than the 650's.
The effect on the Grado SR-60 will sound different again because it has much lower input impedance, and its impedance is pretty much flat. On those, I'd predict lower overall volumes as the M8's impedance goes higher.
A pair of Ety 4s will probably sound shelved in the upper treble. This one I can actually test out since I have a pair of those, and will try out soon.
This is all to say that the impedance switch's effect depends very much on the headphones you're using.