I have not used a ZMF. I don't recall any hiss through my 300 ohm Senns or 600 ohm Beyers, though. Hum is just kinda a thing with tube amps. Mine hums when I crank the volume pot up really high.
You're not compromising, per se, you're just finding a setting you like. The impedance settings adjust the output impedance, which controls the damping factor. The damping factor is the ratio of load impedance (your headphone's impedance) to the output impedance. The higher the output impedance, the lower the damping factor. Damping factor gives you an idea of how quickly the amplifier can stop a headphone/speaker driver from moving, or stated another way, large damping factor allows the amplifier to impart a larger acceleration to the driver. The higher the damping factor the more controlled the driver will be. That sounds great, but there can be too much of a good thing there. Too much damping and the sound can be very dry and chalky. On the other end, too little damping and the sound gets very wet and sloppy. WIth 1AMK2 and 300 ohm cans, the damping factor will be enough on all settings to give you a very detailed, well controlled sound. But, the different settings allow you set to your preference how dry or wet you like it. Apologies if that's too pedantic, but I just wanted to make it clear that you're not compromising and all you're doing is a little bit of tuning to taste. Enjoy!