Anyone ever tried using a power amp to make 120VAC

I think that's what the PS Audio power regenerators -P300 and P500 are doing.

Yes, I am doing it. An audio signal generator can be used as the front end. That will allow you to vary the frequency too. You could just use a step down from the incoming line and filter the output to clean it up before feeding it to the power amp, but the output voltage would still fluctuate with the incoming line.

I'm not sure his 2X reccomendation is good enough. Bear in mind that the power amp will be working a lot harder than it would typically with a music signal so I think one rated at least 4X the load power would be better. It is going to get hot with a the constant demand you are putting on it vs. a signal from a music source.

Hooking up a step down backwards will work but the side that is now the primary will have more current so it has to be rated accordingly. Find a step down that has a secondary current rating that exceeds what it will have to handle as the primary. Let's say you have a 1:4 stepup that outputs 120V and your equipment draws 2 amps. That means the primary side will have to handle 8 amps and your amp has to be able to supply it.

Many years ago there was a modification to the Dyna Kit (yes, that long ago) tube amp to do this.