I never said anything about expectation bias. I have "heard" nonexistant differences that I totally didn't expect to on numerous occasions. I really don't understand why people think the fact that they didn't conciously expect a difference means anything. Also, I didn't say that amps "have no effect on sound". But you're right, I should have been more clear that there are some things amps can do that have a significant impact.
High Z out changes the frequency response in a headphone-dependent way, so obviously that is going to be audible some of the time (though, again, not as much as audiophiles like to think; in particular, planars are mostly unaffected, see below). Any amp with flat FR, low Z out, sufficient power/voltage output, and sufficiently low distortion (say, <0.01% for both THD and IMD), is going to sound very similar to any other amp that meets these criteria (blind, obviously). Pretty much any mediocre or better SS/hybrid amp will meet these criteria (not sure how many portables qualify). When people say this amp is bright or harsh or warm or whatever, this ought to be visible in the FR, because these terms refer to specific features in the FR. I own way more audio gear than is healthy, because I hear differences in sighted listening. However, in blind tests, the differences are exceedingly subtle once you get beyond entry-level gear (say, beyond a magni 3).
The Aeon (mine is closed, not that it matters much) is fairly difficult to drive, on account of low impedance and low efficiency. It needs a fair amount of current drive. Desktop amps are fine, but not all portables can drive it without soft-clipping. It is not affected by output impedance, though, aside from an overall drop in loudness. If you hear a difference in tonality between the A30 and O2 with the Aeons, it is not due to output R.