my setup is a fatar 37 key midi controller keyboard into the mpc midi1 in. electric moFx (which i use as a midi knob controller) into the mpc midi2 in. mpc midi1 out into the computer.
then in reason i can use the mpc pads to make beats using redrum, dr rex players or either of the samplers.
i can use the midi keys to play melodies on the samplers or synths.
i can use the moFx's knobs to freak settings, filters, etc on the synths, samplers, and effects units.
and it works very well for me. very well.