I use a variac for all my old amps and it does make a difference. I recently read some threads by Guitarpicker where he describes this transformer that feeds back into the incoming voltage and in effect lowers the voltage to 110. The nice thing is the parts are cheap and the unit light wieght. He built this into his amp so it wasn't something extra he had to carry around. I bet that is what is in that amp preserver saver box. If you do some seacrhing I bet you could find the threads and they explain everything.
I looked at the instructions... thought about building one and then wondered if I had the experience to think through it correctly the first time. Pics of a finished working unit would help and probably push me to make one. Otherwise I am more likely to avoid the risk and buy one.
A friend whose opinions I trust has one but is in another town and I've yet to see it. He has retired his variac. The Amp Preserver has a large voltmeter built in, has two settings(I believe -6v and -12v) and is lightweight but built like a tank. As the builder makes them to order he had his made with a 5 or 6 outlet strip built in and powers everything through it. HE SAYS that having the proper voltage makes a huge difference in the background noise and dynamics with some of his amps (vintage AC30 and Fenders especially) and is not noticeable with others. This guy has puppydog ears and doesn't buy into snakeoil, but again I have no personal experience.