Stock Etherwave Plus here. My minimal signal chain usually is Etherwave audio out -> Behringer Tube Ultragain MIC100 or MIC 200 (inexpensive clone of ART tube microphone preamps) -> Vox Valvetronix Tonelab ST1 modeller/multi-FX pedal -> Amplifier (Fender Greta 2 Watt tube amp, Torque T100K keyboard amp or Vox Pathfinder 15EXR). The most clean and suitable amplifier models for theremin on it I find to be US Blues Special and Vox AC15 Special, which are both rich in bass and attenuate the high end a little, so that suppresses the 'too electric' higher harmonics to a more acoustic sounding timbre. I also like to use the filtron effect set to its lowest sensitivity, it resembles the effect of the 'Ah'-EHX Talking Machine sound, but the filter varies a little with the volume, so with a raised volume it sounds brighter with a higher filter frequency peak, which also gives it some qualities that more resemble an acoustic instrument.
Depending on the settings on the efects and amplifation chain, the brightness control on the Etherwave is usually not over about 2 o 'clock and often dialed below the center setting and the waveform mostly between 10 and 1 o 'clock. For lower bass or a fattened sound the Vox Tonelab has both an octaver effect as well as a Pitch Shift/WhammyPedal effect. So for a large part I can even get around the 'raspy/moped' low frequency character of the stock Etherwave when I don't want that and have it when I do (for emulations of a purring cat and whale sonar click trains).