Will a transformer, 220 to 100v, ruin the sound?

Listen to Eldartford...he is wise!

Changing the transformer taps on HiFi equipment often works fine, just like Eldartford says. There are some exceptions though. Some high end equipment manufacturers (Krell for example) now install a mains frequency sensing chip to prevent US gear (110V, 60Hz) being freely imported into Europe (~230V, 50Hz) and simply used with a step down transformer (220-240V down to 110V). The equipment simply will not work if you do this (I found out the hard way when I took my Krell HTS to Europe for a temporary overseas assignment). These manufacturers want you to buy the equipment in Europe in the first place, or pay them (many $$$) to do the conversion for you. See my previous post on this subject. Be warned.

I was told by Tom Tutay, a well respected tube amp tech, that a step up or step down transformer would better isolate the piece of equipment in question from the ac line and would thus be a good thing. I never tried it so I don't really know.