The Etherwave Plus is most time not the best choice for the aspiring thereminist:
An Etherwave Plus is nothing other than an Etherwave Standard with some additional circuitry which gives:
- A "Power On" LED
- A headphone output
- Pitch CV, Volume CV and Gate outputs at the bottom side.
The main circuit board which deals with the real theremin aspects is strictly the same in both models, the Standard and the Plus.
The Pitch CV, Volume CV and Gate outputs are only needed by people who want to control analogue Moog Synthesizers (Little Phatty, Minimoog, etc.) with gestures around their theremin. Most thereminists will neither need nor use those.
The "Power On" LED is a nice feature, but it isn't important. The headphone output is electronically realized in a very cheap manner, the sound quality is atrocious and since the Etherwave theremins are not longer grounded when they aren't connected to an amplifier, the pitch and volume fields become highly unstable when playing on headphones. So, that isn't a useful feature, too.
The additional circuit draws naturally more current from the power supply which makes that the internal voltage regulators work at higher temperature which increases the fail risk, the warm-up time, and the thermal instability.
Thus, in my eyes (playing, fixing and upgrading Etherwaves for many years) there is absolutely NO reason to prefer the Plus over the Standard.
The Standard is (at least in Europe) 100€ cheaper than the Plus. Thus, even when adding 59€ for the ESPE01 module, you have a ways better quality/price relationship. And compared to the price of a professional concert violin or a grand piano, the Etherwave is never too expensive!