Confirmation bias is real and very common here. Not saying cables don't matter but when two cables are close, 2 component are close, a new component is added that doesn't make much difference, tubes are swapped, it is easy to get confirmation bias.
I am certain many of the positive reviews here on the forum absolutely reek with confirmation bias.
So do your tests for several hours each. Keep an open mind. go back and forth a couple of times. Do everything you can to keep from being a victim of confirmation bias.
Sometimes I just do the upgrade and don't try to judge it, other than to confirm I did no harm.
Jerry