I use an equalizer via tape monitor so I can switch it off and on via remote control instantaneously. When I change EQ settings (mostly is less than 1 dB change..so we are talking about subtle), the only I can tell if the new setting(s) is better than the old one is if I switch it off and on instantaneously.

If there was any time lapse (even few seconds) were between listening sessions, there is no way in the world I could tell if new EQ settings is an improvement or not.
So from personal experience, when some cable reviewer say that when he/she swap cable wires (which probably be more than few seconds) and noticed 'slight' improvement in midrange, highs or sound stage, I think to my self that this guy might:

1. have a memory like an elephant that can remember every music passages.
2. or he/she is imagining things.
3. or..................