instead of listing flat headphones, it's safer(and more realistic) to assume that none are because we are physically different people expecting a different neutral. also http://seanolive.blogspot.com/2009/10/audios-circle-of-confusion.html in any case, you'd want to measure stuff yourself to try and rule out unknown parameters once you're down that rabbit hole. for a 300ohm headphone it's not really important, because small mistakes, neglecting the extra cabling used to measure, and the amp's own impedance curve, aren't usually going to make any big difference compared to 300ohm and more of the headphone. but when trying to match theory and measurements on lower impedance headphones/IEMs, even my own low-fi measurements fail to be accurate enough to account for everything and give me pretty result strictly aligning to electrical laws. not that there is more to audio signal than electrical laws!!!!!!!! just the like with everything else, it can be hard to remove or control all variables in our circuit. another point which is only my anecdotal experience fooling around with resistors, an EQ, and a mic, along with something I read long ago somewhere that may or may not be true(check that endorsement!^_^): changing the damping ratio for headphones and IEMs, isn't IMO, as significant as it can be on some big fat speakers(FR non withstanding!!!!!!!). if you make sure to compensate the frequency response, the changes aren't all that, subjectively. IMO, it's yet another case of being misguided by how an impulse response may look. if you look at the impulse response out of the headphone, you'll wish to get critically damped stuff all year long, but I'm of the opinion that impressions of differences will come from many other places and shouldn't be assumed to all be the headphone and all be electrical damping. also in the case of your own amp, there are a few out of the ordinary things in it(starting by all those impedance settings), so you can't rule out that the amp itself may change sound on its own in some audible ways, when those impedance settings are switched.