You know, depending on what the piezo puts out, an amp with a fairly open and flat response might not offer the most flattering presentation of its sound (not knocking the Fishman -- looks like a nice system). Too, the harmonic content that the Studio adds to the signal and which is so enriching of slab tone might not be what you're looking for with piezo signal from an ABG (or it might -- I'm just spitballing ideas, here).
I've been sitting out because I don't do upright and have never owned a piezo bass of any kind -- but I've played piezo-equipped basses, so when I read UB's question my first thought was that I was not at all surprised to hear that it would be a challenge to find a rig that would work with both piezo and magnetic pickups, given that they sound so strikingly different from each other -- and that's true with like instruments, let alone unlike ones such as EB and URB.
In short: MarkA, I don't think you are spitballing (for what my opinion is worth).
