So just to be clear. (clarification - here when I talk about "amp", "amplifiers' or "amplifying device" I'm talking about the semiconductor itself - be it any type of amplifying device construction - IC chip, an opamp, a BJT, JFET, MOSFET, darlington pair etc etc)
An amplifying device, inside the metal box that most people call an amp, has multiple inputs - audio signal and power signals. Any signal on those (unless designed to cancel out) will result in noise on the output.
Power AC is simply an efficiency measure for electrical distribution. Amplifier power is 99.9% of the time DC (ie it should not fluctuate). Preventing AC on a DC power rail is what your power supply filters and voltage regulators are there todo.
To compound things further - amplifiers as they amplify draw current from the power rails and thus cause their own signal on the DC power rail. This is what decoupling and power line inductors are there to reduce/eliminate and also your voltage regulators, however this is cost to an amp manufacturer.
Power conditioners are simply a way to create less noise (distortion of the AC signal is noise and creates frequencies other than the 50Hz sine wave of the mains AC power itself).
So filtering AC is irrelevant - what is really needed is clean DC to your amplifying device. How you achieve that (ie great amp power supply or lots of mystical components and active power conditioners etc ahead of that) is immaterial as long as the DC power rail is crystal clear. If you can ensure that the amplified signal noise (that includes any distortions caused by power line noise interacting with the digital/analogue processes) is below 120dB then you're really not going to hear any difference.
Lastly - AC power varies over the day, minute to minute. The AC RMS you know as 230V mains can vary as much as 10%.. my mains can be 253Vrms whereas yours may be below the 230Vrms and never get to that. This means if the power supply is not regulated, two people comparing the same things and mods on the same amp may get a different resulting sound. This is one of the annoyances with tube amps - the amplification process is not linear and as the AC varies, so too does the DC power rail and that then varies the non-linear curve of the tube amplification (ie the function of the input voltage audio signal to the output current varies with the same input). Only when you start regulating the power rails do you reduce that, however then the regulated power reduces the 'liveliness etc' when in reality voltage regulation works fine for audio frequencies but any noise in RF on that line they can't cope with, so noise into audio.. which changes the process .. etc etc.. (and sometimes causes oscillations which can show up in the audio listening as harsh treble - not that you're listening to the RF but the oscillation causes power draw that causes issues with amplifying the treble without distortion).
In short if your amp is designed correctly then you will not need mystical gadgets. And here is the rub - the majority of amps are not designed correctly... (even the posh ones)
