The two ends of the digipots aren’t hardwired to grounds so it could work just like any potentiometer I think? Mixing before the amp on board might be weird, though. I don’t think the mix points would cut off the negative portion of the signal. I’m not sure about the circuit, but you could put a resistor before your signal mixes.
You could also put your signals at a middle voltage reference point with a breadboard, a quad op-amp and a few resistors. The amp removes the DC offset.
What is line level? +-1 or 2 volts?