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.
I tried replying to this a month ago, but I included a link and now the post is stuck awaiting moderation.
I use a hosa CMP-105 cable and get good results.
Awesome. Thanks for the link Kevin. Definitely going to do this.