MB has a mono output. Phono are stereo ins, so choose a mono channel on your mixer, center pan and you'll hear a mono signal coming out from both speakers.
Also, if possible use a mono jack cable to plug your synth into a line-in on your mixer. If still using minijack to RCA's everything has to be mono (jack to minijack adapter + RCA's). MB can add some noise to its main output if volume knob is not set to max, so crank it up and then trim your mixer channel to avoid peaking.
Hope this helps