I most definitely noticed an improvement by running two dedicated lines (one for amps, and one for source/pre). But then, I am running several high current amps (two Adcom GFA-555II's in bridged mono, one Adcom GFA-2535, with two of the channels bridged mono), and the one (non-dedicated) line I had been using couldn't provide all the juice that I needed. While I feel that installing the two dedicated lines was the main improvement, I have also incorporated all PorterPorts, all DIY power cables (from Ernie Munier), and PS Audio Ulitimate Outlets on each line (with a HC model for the amps, and with PorterPorts installed for the two non-HC models). I believe that all of these components have been beneficial to my setup. Also, FWIW, my dedicated pre/source AC line is 12AWG Belden, and the line for the amps is 10AWG Romex (the 9AWG Belden was too pricey for me).
Hope this helps, Tom.