Actually, since this mod removes the effects module, I would suggest hunting down an outboard unit that will allow you to "stereoize" the mono output. And for that, you can't do much better than hunting down a good ol' Yamaha SPX90. These days, they go for dimes on the dollar on Reverb, eBay, etc, and they have a great, character-filled sound and loads of "abuse potential". You WILL need some way to attenuate the output level, though, as synth-level signals are going to be too hot for the SPX90's front end.
One other thing: move the Malekko MIX 4 so that it's more usable with the Marbles. This way, you can also use four of the Marbles' individual outputs to put together complex composite modulation waveforms. Definitely a bonus, and the MIX 4's DC-coupling allows for that to happen.