I was just about to suggest the MU series, and then I saw you mentioned it at the end.. Take a look at the MU's... They're pretty decent sound modules.. Although you already mentioned one unit that I personally feel is great and that's the Motif Rack. It really isn't quite so big.. Granted it fits in a rack width wise, but the height is good. Could easy fit ontop of something..

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.