For the speakers, there was never really a choice to make. As soon as I saw them on LGR's review[1] I knew I wanted a set of Roland MA-12c. They have everything I wanted, from sound quality, build quality, heavyness, amplification, shielding, to being beige.
The MA-12C owner manual indicates that these speakers have three inputs and automatically fixes them. If you don't want to mix in the SoundBlaster, you can just connect both the MIDI synthesizer and the Sound card output into the speakers.
Note that if the Sound Blaster 16 audio out is 3.5mm, the MA-12C inputs are 1.4in. The solid feeling connectors require a Roland Black Series cable to connect both components.
I briefly contemplated using the Checkmate 1500+ integrated speakers but I assumed the sound quality would not be the same. Also, nothing screams '90s like a big double ROLAND written on the speakers!
^ | [1] | Roland MA-12Cs: Best Retro Computer Speakers I've Had! |