Playback

The Casio would have only shown on your MIDI in/out list. It should not have affected the Audio drivers. If it does then don't use it. 
Just unplug the Casio and re boot. Go into properties and re-check the Dou capture boxes if not already ticked. 
 
Home stereo speakers will work for studio monitors in a pinch, just find a used integrated ( home stereo)  amp to match. They are a dime a dozen, Yard sales, pawn shops and thrift shops. Other wise you'll have to buy a good power amp which can run you a few hundred easily. So then your back to square one and might as well buy a proper set of powered studio monitors. 

I take it you are talking about the outputs of the Duo? In that case in a stereo amp, you can plug into "Aux" input, or "Tape Monitor" inputs with adaptors if needed, not phono, that is for a turntable, not Tape Rec because that is to get the signal out of the amp coming into it to - well record with (usually on a tape deck) but that is what it is used for, and video, maybe can be used for just the audio inputs, since you are not using video and have no need to have a TV on to get the video, just use the audio inputs of that, and Right channel is always Red, Left is White, and video would be Yellow if you were using video signal which you do not have to use. Just set the amp to the appropriate input you are using.
 

Right... no Tape monitor input.  Just CD, Phono, Video, Tape MD - Rec - Play.   I'll try the Video input. 
 
Thanks