Why do I need power management if I have a great power cord?

To atmasphere: I’m puzzled by your comment that this Nelson Pass quoted 48 Amps is “how much current is present when the power supply of the amp is shorted. It’s available to the output section in the form of charged capacitance. This is used to help reduce IMD in the output section and is not something that the output section can pass to the loudspeaker.” Perhaps I failed to see what you did there; but I thought that the extended time that my tube amp can drive my speakers from the source when I turn the tube amp off (while the source is still playing) was, in fact, the capacitors discharging through the speakers.

I thought it was funny to use the word 'pass' when talking about a Pass Labs amp...

At any rate, all that extra capacitance used in so many amps is there to reduce IMD at high volumes (so the amp will sound smoother). The amp isn't going to be able to make any more power since the voltage in the power supply plays a direct role in that- to make more current you'll need more voltage. All the excess capacity does is prevent the voltage from sagging over a short period of time when the amp is making higher amounts of power.

Many solid state amps have response to DC. This means that the low frequency timing constants in the amplifier go lower than that of the power supply (unless its a battery)- thus low frequency noise can cause the amplifier to modulate its power supply. When this happens, IMD increases. This can be avoided by having enough feedback in the amplifier circuit, allowing it to reject power supply noise, but that amount of feedback is a rather large value (+35dB), outside the reach of most current (and certainly vintage) amplifier designs. So for amps having lower amounts of feedback (or none at all) excess capacitance is used.

Because that current rating is actually a rating of how much current will flow if the power supply were to be shorted out (IOW nothing to do with the actual full power of the amp) you wind up with some advertised claims of 'current' which are pretty outrageous. The article I linked from a prior post explains in simple math how outrageous those claims would be if they were meant to be the actual current the amplifier is able to make into a speaker load!

Here is the latest video from Amir on power cords.

Here is the latest video from Amir on power cords.

This statement is false. Amirm is testing a particular cable- an Audioquest, not all power cords. It would be correct to say 'Here is the latest video from Amir on an Audiquest power cord'.

However, there is a flaw in his testing regime, which is that all power cords obey Ohm's Law; the conditions he set up for testing did not have a great power draw on the cable. As a result, negligible voltage drop across the cord and quite naturally: negligible results.

The benefit of a good power cord IOW has to do with the current drawn through the cord which won't happen with lower powered equipment (for example, an Audio Precision tester).

Power cords that can make a difference don't have to be expensive however! I agree with Amirm that $5,000 should only be spent if it makes you feel better somehow... IMO/IME there's not much use in messing with power cords unless you have equipment that draws enough power to result in a substantial voltage drop across the cord.

I've seen a power cord result in a loss of 40 Watts at full power from one of our amps (the MA-1). That's a pretty measurable result! But the MA-1 draws a bit of power, being a 140 Watt class A triode amplifier.

Put another way, if you want to get the most out of some amplifiers a power cord can really help, and with other amps maybe not at all. The power that the amp draws from the wall plays an enormous role in this which cannot be ignored as we see Amirm doing in this video.

I have a 300A welder(like it).
However I do not use it for speakers.

And I would not consider a Pass amp for use as a welder, but it might be cheaper than some welding machines.

Can you recommend to me some cheap options? As far as I know, most of them are unsuitable for thin metal and cables

All the best!