I think a stereo connection is only needed if you're also monitoring in stereo. Pre or post stack can both sound good. But the thing is: The location of the loop is stored with the rig, so you always have the choice. 
I'm not sure about the exact reason but I would turn the mix in the Strymon up to 100% and the mix of the KPA loop to taste. Also I would try a mono connection with a mono loop to see if it makes a difference. Try pre and post stack too.
The Mobius has a switch on the back for stereo/pre-post mode, is that set to stereo?
The Mobius has a switch on the back for stereo/pre-post mode, is that set to stereo?
yes it is.
Make sure you are not comparing apples to oranges. If you comparison of it sounding good is in mono, then make sure you set Kemper loop to mono and the Mobius to mono. You said the Mobius sounded good with other amps, which normally don't have stereo loops. Thus you probably used the left (mono) jack on the mobius into your amps effects loop, or front end of amp. In that case, the switch should have been set to mono since in stereo mode the mobious would have sent a different left and right mix. In other words, if the Mobius is set to stereo and you are only using the left (mono) jack you are only sending the left signal out and all the sounds meant to come out of the right side are going no where, since the right jack is unplugged. Therefore, in that set up, you'd keep the mobius set to mono and use the left (mono) jack, which if I believe will give you a combo of what would have been sent to both the left and right in stereo mode. Thus stereo mode is only to be used when you are using both Left and Right jacks on the Mobius, which you can't and aren't with a normal guitar amp.
Now with the Kemper, you can use the effects in stereo, but you need a special cable, and there's no point unless you are monitoring in stereo anyway. However if you were, you need a special Y adaptor cable, with two normal quarter inch guitar plugs on the one end and a stereo quarter inch on the other. You'll know its a stereo quarter inch because it has two white or black rings near the tip on it like a headphone adaptor, not one, where as the two on other end would have one (most of these Y cables are designed this way, but check
However, since you are using the Kemper with just a single monitor, you don't want to set it up this way. You therefore should select "mono loop" on the kemper slot you use. Then use just a regular single patch cord or guitar cable from the Left (mono) output of the mobius into the return jack of the Kemper. and of course, a single regular patch cord or cable going from the effect send of the kemper into the Left input of the mobius (assuming the mobius has both left and right inputs, if only one input you use that. ) Finally set the switch on the mobius to mono. If you don't set it up this way, all your patches will sound different and probably worse than they were intended.
Remember if your Mobius is set to stereo and the rest of your set up isn't wired with the above mentioned set up for stereo, your missing half of the Mobius sound!
