Couple of general hints. Both hi and lo impedance PUs will vary the otuput voltage depending on the input impedance of the (pre)amp they're plugged into. The ballpark starting figure for impedance matching is 20:1. That means that a 100 ohm PU will need a 2000 (or greater) ohm load to be matched. If you can locate the specs for your PUs, they'll tell you the voltage rating into a specific load impedance. That'll probably be their optimum match.
The typical unpowered hi impedance PU as not going to output anywhere near 1V. The last part of your question is a little tricky. You didn't mention whether the piezo is hi(single conductor) impedance, lo(2 conductor) impedance, whether it's balanced/unbalanced, and what the impedance is. If they both have roughly the same impedance and are unbalanced, just wire it up like a regular 2 PU guitar.
SD and a piezo.. that's kind of an odd combo. Can I assume it's an upright bass? I also suggest reposting at the Guitar Forum above. They should be able to guide you along way better than me.
I'd also suggest an e-mail to SD tech. Be as thorough with your question as you can.
Good luck, Paul
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