I think I would look at Logitech PC speakers with a sub woofer. They get high review ratings.
And quite a few arranger keyboard users that perform live use higher powered Logitech speakers for performing live. ( search for Logitech at
http://www.synthzone.com/forum/ubbthreads.php/forums/37/1/General_Arranger_Keyboard_Foru )
But for home probably something like the Logitech Z313 for around $50 would be just fine and would make the Miniak sound great. Plus you could also use them as your PC speakers as well.
Logitech Z313 at Best Buy Of course if you need to go cheaper you could always get a set without the sub woofer .... though they wouldn't sound as good. Whatever PC speaker system you got you will have to buy a $5 ( approx ) audio cable adapter so you can plug the audio cable into the speakers as it is unlikely to be 1/4 inch phono jack that you have coming out of the Miniak ... but not a big deal.