New expensive power cord for amp and no change in sound?

Not buying it. The voltage drop across a few feet of power cable can't be more than a volt. This is insignificant compared to the normal variability of the supply fluctuations between 114 and 126 VAC.

@noromance I've measured over 2 Volts, but that was on a class A triode amp that made 140 Watts... or it did if the AC was right. When the spec line Voltage was at 120V the amp only made 100 Watts. The measured AC line Voltage at the IEC connection was closer to 117V as opposed to the Voltage at the outlet. A 40 Watt loss is not only very measurable but also audible.

As I said earlier, the power draw from the line is an important aspect of what you can hear and measure. Internal power supply regulation as well as feedback to allow the amp (or preamp) to reject effects of AC line Voltage have an effect as well. So some people will hear things and others will not.

This also explains why many people report better sound at night (which the AC line Voltage tends to be higher).

@roxy1927 The reason you can hear changes with power cords has to do with the AC line Voltage drop across the cord. More expensive cables tend to have less Voltage drop.

But how much this affects various bits of equipment is a different matter. Some equipment is sensitive to Voltage drop and other equipment is not. It also makes a difference how much power the equipment draws.

If your amp uses feedback (and the Pass Labs does) then its able to reject to a certain extent the effects of reduced AC line Voltage, despite a high power draw. So if the $500 cable wasn't a whole lot heavier than the stock power cord, you may have heard nothing at all until you reached a higher power level, which might be well above your normal listening habits.

As you may have surmised, not all power cords are the same.