Anyway, the question got to be, should I add a pre-amp or an amp to increase the sensitivity of the scope.
Hard choice, I have experience of building a high frequency constant current Howland based drive for the driver, but my experience with the Howland is that you cannot control the offset and it's very difficult to balance, as in you need some serious precision resistors for reliable results.
I think I could amplify the differential output using some e.g. AD8051 in stages of x2, maybe 3 stages in total for optimal results, then using some 74hc405x to switch various loads and gains for extended ranges, all controllable by the device using the available digital IO.