Clive, yes, you can plug your Etherwave theremin into a guitar amp but it will probably not be compatible with a living room stereo without some attenuation of its output. If you overload your stereo, you could damage it.
What I suggest you do, unless you want to end up in divorce court, is to get yourself an small amplifier with a headphone jack (unless your unit is already provided with one) so that the only person who will be driven crazy by your thereminizations will be YOU!
When you get to the point where you can play recognizable melodies with reasonable accuracy on your instrument, you may want to use your stereo system for playback accompaniment.
Welcome to the Wonderful World of the Theremin!