Question for y'all tonight, I've been wondering this for a bit. Multiple times I have come across someone stating that having a high dampening factor with ZMFs and AMPs should be avoided like the plague. Amps with less than 1 Ohm output impedance not making friends with ZMF or other 300Ohm cans. Can anyone convey some truth to this?
First and foremost, use all rule of thumbs as guidance, and break them at will. Some of my best discoveries have been just sticking things into other things and seeing if I liked the sound. Always trust your ears. And everyone’s ears, and tastes, are different.
Second, the general rule is that you want to have the right damping ratio between the headphone impedance and the amp’s output impedance. What that is varies. The golden rule most often quoted is 8:1, but 3:1 can be great, and sometimes 100:1 can be great too. It’s important I understand what happens and then play with it. The higher damping ratio, the amp has more control over the driver, but at some point that can start to manifest itself in a sound that is too tight, lacking a sense of natural decay and resonance (those harmonics we subconsciously find pleasing). When under-damped, those may become too present, and the sound may feel a bit loose and lacking crispness. Where you like to be on the spectrum is also a matter of taste.
Other than that, consider that taking 2 amps that have an identical output impedance with the same headphones will still yield 2 different sound signatures. So that is only one element that affects synergy. The amp’s circuit design, choice of power supply, transformers, capacitors, etc. will all affect its sound, and in turn its synergy with your headphones.
That’s why rules of thumb are helpful, but aren’t a hard rule.
That said, I personally prefer using an OTL amp with my high impedance dynamic headphones, where I find I get the damping sweet spot, and with not output transformers in the chain the over sound for me is the most pristine and beautiful. And I prefer using SS, Hybrid, or TC amps with planar headphones for that power, delivered with great control across a lower output impedance.