Below are workable suggestions, if you have lots of money there are many ways to do it.
Any amplifier can be used with the EtherWave Standard if adapted properly. It is always better to have too much signal so it can be reduced to a proper level.
Audio Level Reducer Amazon $10
RCA to 1/4 Adapter Amazon
RCA dual mono to 1/4 Amazon I would start with this at the EWS and work my way outward.
When using headphones you will need a headphone amplifier, this one has RCA inputs on the back side so attach to Level Reducer Amazon
When using headphones you need to keep the theremin earth grounded so use a Y adapter plugged into your EWS so the main earth grounded power amplifier stays connected to the theremin whether on or not. Amazon $6
If you overdrive your main amplifier input and are unaware of it this will definitely improve the beauty to your sound.
Christopher
Edit: I posted general information about the EWS not the plus. The thread addresses newcomers, have you become another Debbie Downer Dewster. From anyone giving advice listen to there sound samples to know their understanding of the theremin.