With most DAC preamps, that would be an easy question: set the volume to max, or as high as possible. That's because as you turn down the volume, the SNR drops with it. The max SNR is at full volume. That's because most preamps have a fixed gain ratio and the volume knob just attenuates the signal.
But the Soul has a totally different design. Its volume knob changes the analog gain. That's why it clicks as you turn it; that's the sound of relays clicking on and off as it swaps the resistors in the analog gain-feedback loop. The net effect is that as you turn down the volume, it reduces gain, which means the SNR stays very high. Benefits of this (A) SNR remains high at all volume settings, (B) perfect channel balance at all settings, (C) lower noise (no potentiometer in signal path).
So what volume position gives the best output depends on the Stax amp. It looks like it has an analog potentiometer volume control. These usually have the best sound quality and channel balance in the top half of their range. So a good place to start might be to set the Soul volume knob to whatever position puts the Stax volume control around the 12:00 to 2:00 position (maybe even max, depending on the Stax amp's gain and volume knob). Adjust from there using recordings with very wide dynamic range.
Note: this is the opposite of what one would normally do, which is to have volume maxed close to the source, then set the listening level using the volume control furthest downstream. This is due to the Soul's different design. Nobody listens at full scale volume. Whatever level of attenuation you need, say -16 dB or whatever, the Soul is likely to give you that attenuation with less of a reduction in SNR, than the Stax amp. Of course, if the Stax amp has audible hum or sounds bad at full volume, turn it down until that goes away.