Hi Jim, I needed to experiment as I don't use the input of the MT so it was nice to actually do it.
'But there's no level control for the external audio in? or does the minitaur just expect you'll set gains tructure via the output of the other synth/sampler? '
Since there is no input gain you have to drive your signal rather hot. It's fairly quiet and unfortunately there's no telling when you're actually scraping the ceiling. Pretty much nothing happens if you don't press any keys so you'll have to use a midi keyboard in order for the sound to come trough.
'Does that audio in replace an oscilator and the mixer control for that socilator takes over as level for the audio input?'
You have to turn off the OSC volumes and then you're presented with the external sound. If you leave the volumes up you'll be hearing double content (which could be nice if you're designing or layering stuff). In this experiment I drove a Roland TB-03 throug it and have to say the filters eat it up nicely. There's nice bottom end and or screamy resonance but again, you'll have to drive the signal in loud as I found the input to be rather quiet.
All in all I love this little box. It's my foundation in the live sets I do but I wouldn't buy it just for the ex in. If you're covered in the mono synth dept then I'd recommend you sticking to your guns. Then again, one could never have too much gear, right?