Specifically, I'm refering to a mismatch between amplifier output, and headphone resistance. My headphone amp is a SPL Auditor with a 9ohm output impedance, my soon to arrive new headphones are Denon AH-D7000s, which are 25ohm and considered to be sensitive (around 106-108db). I have read a great deal of information here on head-fi, and in other places and the information is mixed as it relates to just how much actual impact the mismatch in my equipment will actually have, meaning that I can and should hear fairly easily. For me if it is measurable, but not audible/perceptible then it isn't very important. I have heard from people who say they can't hear any negative impact from the type of impedance mismatching that I am going to be dealing with, while others suggest that the science guiding the impedance question means the sound will be impacted in such a way that I will hear the effects.
I think what is most commonly presented as the impact of impedance mismatching is that the bass response will be less well controlled, and that a higher overall distortion in the sound is likley. My question I hope can be answered outside of the realm of pure measurement, in other words will I for certain hear the negative impact of the mismatching or might it rear its ugly head in such a subtle fashion that I might be able to perceive it, but I would have to be trying hard. From what I have read the mismatch between a 9ohm output and a 25ohm resistance isn't very high all things being considered, and many people in similar situations to mine even suggest they love the combinations! From past experience I had a Valhalla and HF2 combination that I thought sounded quite good.
So I want to again ask, am I for certain going to be enduring significantly degraded audio quality, or is it actually a subtle, and perhaps not even perceptible effect? I have read the theory and it still seems somewhat ambiguous as it appears that the individual characteristics of the headphone matter, and that the actual impedance of the headphone varies throughout the frequency range, meaning the worst possible effects of mismatching aren't always going to be as pronounched. Any thoughts would be greatly appreciated. Thanks.