So, the MXR M87 arrived this morning (thanks Alex_1_2_3 for lightning fast delivery!) and I have spent some time playing with it.....
As I was setting the unit up, I realised that I could use the input gain LEDs to provide a visual indication of the pickup's relative outputs. A quick pan between piezo and mag revealed that it is definitely the mag pickup that causes the output imbalance, the B, E and A strings generated far more energy, so I have spent some time adjusting the angle of the pickup to balance the strings more evenly. It isn't perfect, but it's a lot better, and it means I can use the compressor to clean up the output in a subtle way. Job done I think.