Well, I hit a hot button for discussion it seems. Do mean this to be a "learning" experience, not arguments. Sorry for any flames going any direction on this, even if I'm just a catalyst to it all!
First order filter should be 6dB / octave not 3dB. It is a VOLTAGE amplitude drop. So it goes 6, 12, 18, 24 dB ETC as you gang filters in series. So, you have to be careful to use 20LOG and 10LOG in the right places. I was thinking optical cable half POWER bandwidth point, which is measured at -3dB. Wow, sorry about that!
"These effects can be quite measurable!! For example, I have seen a 3-volt drop across a 6 foot power cord cost a tube amp of about 35% of its total output power. If you want a reason to look for, that one is pretty basic!"
No, a power supply cord is in SERIES with the ENTIRE ROMEX lead and all the way back to the power station. An insignificant voltage drop and even less so if the lead is equal to the ROMEX in AC resistance. So if you use a 12AWG cord you're NOT going to see appreciable voltage drop across the cord. Do a voltage divider rule to the ENTIRE DC circuit! The AC circuit could care less about what we just paid for the cord. What you WILL find, is CRAPPY wall and IEC sockets that are independent to the cord used. THAT is what you measured. The wall outlet has a limit of 20 amps, no getting around it. You can only get so much continuous DC from that. LARGE filter caps can leverage power gained over time, to the SAME net power over a shorter time. Some call this dynamic power or "high current". But the TOTAL power over time has to be the same.
All things being the SAME (dielectric and conductor size and type - stranded or solid), a 150-ohm twinaxial (two coaxial in parallel) will ALWAYS have half the capacitance per foot. The shield is taken into account when you set the insulation thickness or, you would NOT have a 150-ohms cable would you? Of course not. Yes, if you buy a 120-ohm twinaxial cable the capacitance is not half, but it isn't apples to apples on the "root" insulated conductor. And, the conductor spacing will be twice the spacing or MORE with the same root single ended conductor impedance.
RCA leads do NOT "shield" magnetic "hum" at all. This type of interference is diffusion coupled clear through the shield and non-magnetic (foil or copper) shields. So, to get rid of magnetic coupling requires a TWISTED pair and differential mode transmission with common mode rejection - sometimes called CMMR. The "shields" on audio cables are RF only, and really do very little at that since most inputs use a RF filter cap to ground to strip off RF. It's more show than go at audio. That, and it is CHEAP to do over balanced lines. Power cord have shields to mostly BLOCK crap going INTO the A/C grid FROM your equipment (PC monitors ETC). Collapsing electric fields across a resistive connection throws off RF (lightening and AM radio). Magnetic lines cancel at ninety degrees, but it's impractical to be always ninety degrees and go "forward" to the amp! Sooner or later, you have to turn the cable.
The only other method to block AC hum, is to use ferrous or "magnetic" type materials that intercept and reroute the flux lines around what's inside the shield.
Poor audio circuits can be swamped out with RF, which is terribly inefficient to amplify. So terrible sound can be rooted in RF leakage into the gain stages, clipping the circuits and throwing the audible band into chaos. Super wide band amplifier stages can simply amplify the RF, and NOT clip the audio levels so much. It doesn't matter where clipping happens, it goes all the way through at the audio level after the stage.
Still, I'm NOT a taker on amplifiers ideally needing to be "warm" to sound good (The P/N junction are actually HOT as heck by nature, and run better when COLDER. The new Macintosh amps are set-up to be COOLER running to IMPROVE reliability AND sound. Sure, old school amps are biased to be hot since they didn't take the time to design around a COOLER thermal equilibrium, and the amp was ONLY at design attributes when it was warm. But, THAT point is static after about twenty minutes or so. Transistors like to be cooler to work well, and last a long time. COLD is good, it's just that the DESIGN can not keep the circuit cooler that FORCES a higher thermal stability inflection point. Designers don't WANT that. There isn't a SINGLE attribute on a transistor, inductor, resistor or capacitor that likes HEAT to be better.
You guys and gals go ga-ga over your low loss leads, but never give to the notion how inefficient a HOT transistor is? HEAT is HIGHER resistance everyone! That means you need MORE gain to offset the HEAT, which leads to more circuit paths, and LESS purity of the signal. Sooner or later, the HEAT offsets the GAIN, and the amp design collapses in on itself.
So what you "hear" is an amp that need twenty minutes to be at the DESIGN values of all the components using contemporary (older?) circuit designs.
As for 24/7? Sorry, I don't buy it, and my ears don't buy it and I don't want to buy the electricity to buy it! After twenty minutes my DNA-225 is SILLY hot. I'll turn the radio on for twenty minutes and catch the news, than listen to music to forget all about the news!
Oh, I still contend that speakers, cable / amps are heavily interrelated due to back EMF and how the amplifier's output stages are effected. The amps damping factor with respect to frequency, complex amplitude and phase cancellation does indeed alter what you hear. Each amp likes a different cable to alter the negative effects of the speaker, and itself. But, you can't change your amp or speaker, so we change cables. Some argue (DynAudio) that speaker leads should be CONSTANT impedance at audio. But, some amp and speaker combinations may not like that "ideal" world since the amp and speaker aren't ideal. No ideal LOAD (the speaker) throws a signal BACKWARDS to the amp! So much for ideal circuits.
The pre amp leads, not so much. HIGH impedance terminations and LOW current don't interact as badly. The do have "issues", (there is current) just not so much.
So of all the discussion, I 100% agree on speaker leads contributing to sound (I can hear it). Power leads and pre-leads, no, not so much. I haven't heard it. And, on pretty good stuff; Benz Micro Ruby 3 with Vandersteen Quatro SIG wood series II speakers.
Good stuff everyone.