My using PWM for volume control opens the door to many other possibilities of control.

I think using a preAmp/equalizer is good to balance out the audio level of the playing range.  The PreAmp extra stereo channel can be used as the ear bud pre-amp while using an inline volume control on the bud.

Schematic Corrections:

Phoenix Schematic PDF
Drawn by Don Parrish-Bell
9.19.13

R7-? The board indicates 1K, I have found 470 is better for near the loop silence. If you like your volume response don't change it.

C19-? on volume should be 100pf as C17 is also 100pf.

C10-470uf is on wrong side of power diode. ?

C24-47pf  ? missing, R29 2K2  ? missing

Output-2b & Out-2b share the same connections unless custom modified to use Out-2b for another purpose. ?

The wiper on Pot-2 should have a direct connection to the P side of the transformer. ?

Don, front panel line level adjust with 1/4 jack and using the amber LED with the switched ground for mute indication is a nice touch. Maybe indicate switch as DPDT?  

This section above is drawn incorrect, see corrected below

Ideas:

When it comes to sound very small changes to the wave shape can have a big effect to what you hear.

Pot-2  This effect is very subtle and may not be noticed unless Pot-1 is delivering enough signal through U1b to drive the Opto.

R29-2k2  might be a good spot for a 5K linear Pot to blend/balance J2 output with Audio-1 & Audio-2, the current value is the best. Increasing the value of R29 will decrease the output amplitude slightly but the best sound is what we are after.

R29 is a low impedance spot so a twisted wire pair should work fine.

The Opto modulating trick came about from my first years of experiments, beside the phase shift I noticed that audio on a LED talking to a photo-transistor properly balanced created a more interesting sound.
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Notes of Interest:

LED-1 Green, this moves the Vactrol circuit cutoff voltage 2.1v  higher for better quieting response. Any higher volume will not completely cut off.

R32 at green LED-1 spreads out the quiet shading near the loop. To try this is best if R32 is switched in or out. This will not work with a poorly responding volume setup.

Fixed RF oscillator R27-1k creates better thermal drift balance, C8 keeps cold side of coil at RF ground. This R27 could be jumped to compare if having it is any real effect.

The best ground loop filter I found to place between the theremin and amplifier is the Absolute PNF 200 but it still attenuates the low end (<100 hz) slightly which encourages the use of a pre-amp/equalizer to bring the low end level back up.

Audio-1 is standard sound     

Audio-2 harmonically enhanced sound using T1 & V0618a.

Mount -> R29 & C25 with longer leads (near U3) so one side can be clipped to experiment if its effect in or out of circuit is better. The sound will quiet without these but it may give a more harmonically rich effect? The audio signal from the 3 x 3 oscillator board must be driving the OpAmp U1 strong enough to make the magic happen. Ideal is audio greater than 2v p-p at U3 pin 1.