I'd forgotten about Max. The SFX sound custom page does list a double bass interface which seems to do exactly what we both want - one piezo, one switchable piezo/mag, individual level controls and a phase selection switch. Max's stuff is of the very highest quality (I also have a microthumpinator) but I'd guess that we'd be back somewhere round the 300 quid mark! Be quite a challenge to shoehorn it into a 1590 enclosure.
I have an LS-2 and a Shadow piezo pickup, so I'll give it a go and see if it's useable.