that´s completely wrong for sure and shouldn´t even work .Please check your mackie manua onl how to connect your gear.
Yes I agree, it just does't make sense at all.. when I start with a Line UX2 its always about plugging into the line ins, I will see what I can get from the manual, thank you
I feel your frustrations buddy i too am going through a very similar scenario (im using SPDIF) sorry this reply aint a solution for you just letting ya know you are not alone ?? hope we both can get a resolve soon.
Thanks Adam! Before I post this I saw few similar posts regarding how to connect the interface, its been a struggle for many I think because we just never get the concepts right, most of the time when it works then I just let it be and never wonder why. I feel like these are some newbie questions but Im more afraid if I don't ask I will never learn 
I will try digging more posts and wish you can figure out the solution too, cheers.
Are you saying you connect the Kemper’s outputs to these? I think the desk would be trying to drive the Kemper, as well as the speakers in parallel.
Thats a perfect description for whats going on right now I believe!
Basically yes, in short I connect speakers through XLR outs (rear of interface), usb to pc to get sound from my computer.
I connect my kemper with its TS L/R main outs to the TRS outs I mentioned, right above eq panels, next to footswitch port.
I think first thing I need to know is the difference of whats going on for my guitar signal, by going through the main outs (that sounds great but cannot control volume), or through the line in channels (that I should plug into, but sounds bad now).
I pressed the ground lift next to the main outs but still no luck 
, thanks for explaining so much to me still, perhaps at one point we can prove that its just sth wrong with my equipment (they are with me for quite some years) and I should start to get a new interface haha..
Thanks Edward for the useful info!
Sorry I didn't make it clear I was not plugging into my audio interface's ST return, but "Main Outs L/R" literally, the ones that's just next to the footswitch and phones ports.
I followed your instructions plugging into ch 7/8, checking on the OL getting things right however my tone still sounds fuzzy and "broken"... Not sure if the problem is my tone so I tried using different tones including the default ones, regardless of clean and distortions its still fuzzy and crappy like my signal is still too hot but my kemper input output is always green light. To provide more info the fuzzy noise is still there even if I lower the track's fader, for gods sake I start to wonder if it's just my speaker's problem 
So you can see why I'm confused because at this moment it seems that I could only plug into the Main Outs, however there's no gain control nor I could use the fader to increase the volume, I just couldn't figure out what went wrong 
Hi fellow kemper pro users just few conceptual questions...
I have always used the TS main outs from kemper to connect the main outs (L/R) in my audio interface (I use Pro FX8 if this matters), my guitar tone has always been great but I could never raise my volume further through the faders in the mixer but through my kemper only, so out of curiosity I switched to connecting one of the track's line-in L/Rs from the interface instead.
However my tone became all fuzzy and weak, even tho I could use the fader to increase the volume, Is it because of the built-in EQs that messed up my tone?
So does that mean we should always (or mostly) connect our kemper to our audio interface's main outs when recording or playing?
If so how could I raise my guitar volume through the audio interface instead of raising through the kemper (because I don't want to make the guitar output signal too hot) ? Or am I not using my kemper properly?
Not sure if its a technical question on digital recording but would really appreciate if someone could clear the concepts for me, cheers and stay safe!
