Not finding much difference is a good sign, at least initially, since both amps should produce the audible freq's from 20 to 20 khz pretty evenly. So I would recommend as a test to try them side by side very late at night when it's extremely quiet, or get to some place that's ultra-quiet, then listen to the higher-priced amp through a whole song and then do the same song on the lower-priced amp. Repeat if necessary, always starting with the higher-valued amp. If still no significant difference, change songs.
I'm working on the theory that when people report a really major difference, especially a strong difference in a particular sonic character such as bass or treble, that they are listening in an extremely quiet place where they can hear a pin drop and it sounds like a mallet - and then what they report is exaggerated compared to what you would hear in normal listening. Or, the other possibility is mismatches somewhere along the line.
EDIT: When comparing headphones, I always start with the "better" one if that can be determined, then switch to the lesser one. That tends to show up deficiencies in quality that aren't as obvious the other way around. And it's not psychological - it's just that the missing subtleties are more obvious than going from missing to inclusion, when the differences are subtle.