Pbb, yes, i am sure of what i stated. One is likely to run into severe voltage sag when the circuit is overloaded. Overloading a circuit could occur because:
1) you have too many items plugged into that circuit with all of them turned on
2) you have a single "hungry" device that pulls LOTS of power on a dynamic basis and the power source can't feed it fast enough on demand
3) there is a long run of wire from the main that is too small of a gauge to pass the required amount of current on a continual basis
4) you have a resistive connection somewhere along the line
Physical signs that you are loading down that circuit ( and sometimes even the mains of the entire house ) are when you can "modulate" or "flicker" the lights in the room or other sections of the house to the beat of the music. If the lights dim out upon "firing up" of the amp, that is also another sign that the supply to the amp COULD be "choked" when standing on the throttle.
Under severe conditions, you can even experience "brown outs". Some amps, primarily those with switching supplies and fancy "digital tracking power supplies" may shut down all together or start producing MASSIVE amounts of distortion.
Since many of these conditions are due to sub-standard wiring that is either "antique" or was never installed up to code, running heavy gauge dedicated lines will resolve most of the problems mentioned.
Besides all of this, there are outside variables included. Power coming into your house DOES vary in voltage. If you doubt this, try measuring your voltage at 2 am and then try measuring it again in the middle of the day. This is especially true when there is a LOT of draw taking place, like when it's 100 degrees outside and everyone has their AC turned on. Not only can the voltage drop, it can vary sporadically as the load varies from thermostats kicking off and on at random.
As to a manufacturer making comments regarding their equipment NOT needing prior filtering, ask them:
Do large filter caps reject all frequencies above or below 60 Hz ? The answer is NO !!! This means that any AC based signal that made it into and past the transformer is now going into the circuitry.
Do large filter caps actually filter all frequencies and types of interference equally ? The answer is NO !!! As such, the equipment may be seeing actual harmonics that the power company itself has generated along with anything else that has found it's way into the wiring.
Can RF signals ride AC mains ? The answer is YES !!! With the length of wires used for pumping AC around the country, they make up what may be considered "the longest long wire antenna" in the world.
Is RF interference an AC based source of noise ? The answer is YES !!! As such, it can pass through a transformer basically unhindered if of high intensity.
Can an AC signal pass through a capacitor and / or rectification circuitry ? The answer is YES !!! RF is AC based and can EASILY "ride" or be superimposed on top of DC.
Can DC pass through a transformer ? The answer is YES !!! This lowers transformer efficiency DRASTICALLY and can introduce other types of distortion to the signal.
All of those are reasons why MOST equipment benefits from "extra" filtering prior to the power being fed into their chassis. Since the mass majority of manufacturers don't go to the trouble or expense of designing filters with sharp slopes into their power supplies, some noise and interference IS passed on through the power supply and into the circuitry. Sean