Roland - MPU-104 and MPU-105

Taken separately, the 104 and 105 are useful enough, but taken together they would allow you, for instance, to have a flexible five-instrument-plus-sequencer MIDI system, with any instrument able to be switched in as the recording instrument, and all for a fairly modest outlay. Alternatively, you could set things up to allow any one of five MIDI instruments to control up to another four, with the exact configuration alterable almost as quickly as it takes to figure out what's going where in the first place. *Sold together as a pair*