Huh? I thought you said the sub only has one RCA in and out. How did that turn in to L + R outs?The only way I can think of doing it would be to go R&L out of the DAC into the sub. Then out of the sub R RCA to 1/4 or XLR to R Mackie then L out to 1/4 or XLR to L Mackie. Would this work?
I wonder if this would even sound that good?
Did you check the Mackie's manual? It has RCA inputs as well as 1/4” and XLR. It also says in the Signal Inputs section on Pg. 8: “You can use the remaining [i.e. unused] jacks for “Thru” jacks for daisy-chaining to another monitor or amplifier.”
So: DAC out to Mackie. Then Mackie out to the sub. Piece of cake. It would probably be easiest to make one of the cables RCA to 1/4” TS instead of XLR.