I've read this thread and each poster has his/her reasons for buying in or not on the power cables. JMO but had an experience that confirmed for me, in my system, that they do matter.
Purchased some speakers, Evolution MM3s. Decided to get the Evolution power cords for the powered woofers. You know, recommended kit yada, yada. Had some Fusion Enchanters for my Monoblocks, ribbons. Well the Evolutions were 6' and the Fusions 5'. Basically had to switch as the run to the speakers was easily reached with the Fusions cords and needed the longer cables to reach the amps. So I flipped them. Significant difference. Didn't expect it, didn't want it to be so. Therefore no expectation bias. The difference was as plain as day.