"Modified sine wave" would be more accurately called "modified square
wave" because it's made from two square waves out of phase. The phase

Well I should been more correct

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The waveform is compatible with electronics but it may induce loud
mechanical buzzing.

Would torroid transformers in the amplifier cause problems?

I worry that the transformer would overheat, or that the square wave will cause
transients inside the amplifier. Or the filter and rectifying will screw up.

I don't know what kind of AC power you have in Sweden, but the US has a
neutral line that is live in most inverters. US electronics is designed
so that capacitive and resistive coupling happens more on the neutral
line than the live line. If anything touches the low voltage side of a
crapy inverter, the AC neutral caries a square wave that can damage or
interfere with some electronics.

You mean that between low voltage (-) or (+) and AC-neutral wire. You have full
AC power?

In sweden we have 220V AC 50 Hz. Which is in turn gotten by connecting one
wire to neutral and another one to a phase. Main lines are three phases at
380V (upgraded to 400V now?) which is the phase-to-phase voltage. Phase
"skew" is 120 degrees (2pi/3 rad). Distribution system is PEN.

A true sine wave inverter would be much better to use for a stereo. It
won't induce buzzing and those inverters are more likely to provide an
isolated output.

But there's an issue with additional weight.. not that amplifier is light ,
but still. And ofcourse more or less the double price. I have considered to
build a IBGT pwm style AC inverter.