First note that there are 2 different effects in the SPX: harmonizer and reverb. You may want to control them seperately (I don't remember if they're used at the same time).
As the spx volume seems to be already controlled with cues and the spx_vol_db message (isn't this enough?), you shoud add gain~ objects to the outputs of the effects in the p AUDIOo/p SPX patcher and connect them to a ctlin.
IF your idea is to ask the cellist to control the volume, I don't think it's a good idea as he obviously doesn't sit at the right position to make a good mix.