Fred, you are right saying that this is normal behavior. But your "analysis" of what the LM13700 does here is unfortunately not fully correct.
The inputs of the LM13700 (pins 3 and 4) are first of all connected to the bases of a differential transistor pair. As each differential amplifier without negative feedback, even small signals will drive this diff amp into saturation, clipping will occur. That's how waveshaping is basically done in all Etherwaves, including the PRO. The waveform pot allows to add a small amount of variable negative voltage (from -0.25 to -0.5V) to the (-) input which allows to play on the symmetry of the (initially symmetrical about +/-40mV) clipping. The input signal at the (+) input is a full rectified sinus which goes from 0V to -0.6V.
Now these diodes which are connected from pin2 to the (+) and the (-) input come into play. When there is no voltage at pin2, they have no effect at all (but "zenering" when the input differential voltage exceeds +/-5V, which never happens in an Etherwave). If you make a current flow into pin2, there will be the same bias current through both input transistors, reducing the gm of the differential pair and thus "linearizing" its operation which makes that the clipping limits raise from +/-40mV to up to +/-200mV, making the output signal less clipped, thus softer and rounder. That happens when you turn the brightness pot counterclockwise, making more current flow through R27 into pin2. At the other extreme (brightness fully clockwise) there is no biasing and thus the most sharp clipping.
With the absence of negative feedback, the differential input resistance of the OTA is relatively low which makes that changing the offset voltage on pin4 and the current into pin2 will have an impact onto the signal on pin3. Thus it is normal that the waveform at the raw audio terminal pin varies slightly with the waveform and brightness pot settings. In practice this has no noticeable effect on a tuner which is connected through the ca3080 signal former stage as suggested by Bob Moog.