There won’t usually be a big noticeable difference running a clean signal direct into your mixer vs with RIP other than level. TRS is only needed if you want to ensure less noise when using a longer cable run. Lift is there if you are getting hum in a situation -flipping it may make it go away. Think of RIP as a simple DI box. Level matching, noise, and ground loops can be frustrating things to deal with when you play live.
Regarding the distortion, when I first got mine last week, I ran a sine wave through channel 1 on my triatt in a similar way and added offset using channel 2 and 3. The 3rd out/sum went to RIP. I saw harmonics added using RIP into my motu interface via the FFT analysis software it came with. I believe (but haven’t tried this) you can run a signal through a and out of b back into your mordax to see this. You’ll want to monitor the signal post RIP however you do it.
The signal does thin out as the offset gets higher. There is a sweet spot right before the volume drops out where the signal is nicely overdriven.
I’m very happy I got one and planning on playing a show without my mixer at some point, using RIP to handle levels out of my system.