See, this must be why civilization is in trouble, if that's considered trolling... cause I wouldn't say that's trolling at all, that's a reasonable Q.
I'm also like the other guys: heard the report of hissing/cracking/higher output impedance, went in with a confirmatory bias, but simply did not observe those behavior (at least not yet).
My tiny, uneducated suspicion (i.e. you're more than welcome to diss it) is that due to the metal body or circuitry, if your headphone jack is carrying a little bit of static electricity, when you plug it into the M8 you would see more of an effect than with other phones. Other possibilities are myriad: quality control variances or whatever.
I also tried my M8 with a DAC. You know what, it sounds cleaner than my previous Sony phone on the same external DAC! The M8 sounds cleaner, louder than that phone on both that AND non-aptX Bluetooth. Can you blame me for trashing Sony so much, LOL! But this time my M8 had a constant high-pitched tone coming out the left channel that I haven't been able to troubleshoot. So all I can say is (an alledgely very un-scientific) YMMV.