I use a 3.5mm TRS to 1/4 inch TS for this and get good volume. I’m pretty sure the input is mono and unbalanced, so using a TRS would make it quiet. I would think you could get good volume from using a TRS to dual TS splitter, though. But maybe try the cable I suggested.
This is the cable I use – https://www.sweetwater.com/store/detail/CMP105–hosa-3.5-mm-trs-to-1-4-inch-ts-mono-interconnect-cable-5-foot
