MPC REN Assignable Mix Outs - SP404

I'm running MPC Software version 1.9.6.

Hi, I've searched this topic, but haven't figured it out yet.

I've been monitoring my MPC Ren from the master stereo outputs, into chan 1-2 of my RME Fireface 800.

What I'm trying to figure out , is how to go out from the Assignable Mix Outs from the Ren, into SP404.

I'd like to be able to route that sound back into the MPC Ren, so I can sample sounds with the SP404 fx on them.

I also want to play a beat, go through the SP404 fx, back into the MPC Ren, and out into my DAW or interface.

Can anyone please explain how to do this?

So I'm able to monitor out of 3/4 on the MPC Ren, into the SP404.

I'm using a TS-RCA Cable, to go from MPC into SP404.

I'm using an RCA-TRS cable to go from the SP into my interface (RME Fireface800).

It sounds terrible. Very quiet,

When I try going out from SP404 RCA - into Interface TRS, I only get sound from one side.

I've looked online, and there doesn't seem to be such a thing as a RCA to dual TRS cable. It seems like I should be able to hear left and right if I'm using an RCA - TRS cable, out from SP404 into Interface.