An amplifier meant for guitar may be designed to overdrive/distort sooner when the gain/volume is dialed up, and the signal from a theremin is usually stronger (line level) than from electric guitar (instrument level) that needs more gain, so the dials of guitar amps give very little dial room between silent/quiet and (too) loud and unintentionally distorted.
Some people, including myself use some type of pre-amplifier, actually using it as an attenuator, like a (passive) volume pedal, an ART tube preamp or a Behringer clone thereof directly after the theremin, beforing going in (guitar) (multi) effect pedal(s) and/or a guitar amplifier.
In many guitar tube amps that stock go into overdrive soon, one could replace some or all of the higher gain pre-amp tubes (usually 12AX7) with a pin-compatible lower gain type. I've done such tube-swaps in both my Vox Lil' Night Train as well in a Fender Pawn Shop Greta (both are small tube amps meant for guitar, only a few Watts). It's also quite easy to make a fixed or variable passive attenuator to adapt the theremin's signal to a guitar amp.
Keyboard amps are designed to expect a line level signal, and to sound clean, so they usually work without issues.
Bass guitar amps can work well too, they usually can be set up to handle a stronger signal without going into overdrive/distortion (also for bassguitars with 'active electronics').
