The problem would an impedence mismatch. If your synth has 1/4" output sockets, they are most likely hi-impedence line-level outputs. For the best signal response, you want to connect to the high-impedence line inputs on your converter. XLR connections are low-impedence (mic level) connections. Typically, you use a direct box (or other in-line barrel convertors) in between high-impedence line level connections and low-impedence XLR connections to correct for the impedence mismatch. If your converter has combo XLR-1/4" connectors with the 1/4" line input in the center of the XLR connection, that is a high-impedence line input. Just connect your synth to that.