Originally Posted by colonelkernel8
No, because speakers actually make a very audible and very measurable difference.
Agreed. My background (Biomedical Engineering) isn't in this field, but seems to parallel the concepts that should be at play. Medical signals deal with more noise (1:100 SNR isn't abnormal), and need to be processed much more precisely as poor frequency response, signal processing, and noise attenuation can mean death, not poor SQ (Loved our society motto: When we mess up, you die
If you considered the passive properties (R,L,and C) of a "high-end" and a decent power cord, then pass the signal through transformers, rectifier bridges, and capacitors to obtain your DC supply voltage, the differences between the two cords are less than negligible. The cord does act as a line filter, but hardly has any effect relative to the rest of the electronics that will affect your signal. An educated guess on the order of magnitude of (generous) improvement would be about 1x10^-5 V at the power supply output. That's before signal processing, amplifier common mode rejection ratios, output stage filters, etc- not to mention the fact that increasing the power supply voltage to an op-amp has absolutely no effect (gain is gain is gain) on the output unless you're saturating the op-amp.
The quantum mechanics of electrons are unimportant here, the RLC differences in the power cords, though they do exist, are far less than normal biological variability (including placebo and all other physiological variability, normally assumed @ 20% variability) and very far below the (well studied) difference threshold of the ear. If you can hear the difference between two power cords, it's because one is very poorly shielded/conductive.
So IME if you hear a difference, your $150 bought you a placebo effect. That doesn't mean you don't hear
a difference, it just means the improvement isn't taking place in the electrical domain but in the physiological domain. And I'm only talking about about power cables, the effects of interconnects and other signal cables are orders of magnitude more significant than power cables. They actually have an effect on frequency response (power cables do not), and the noise in a signal cable is amplified (and filtered, but no filter is perfect) for interconnects, or unfiltered if a headphone/speaker cable.