I went back to Guitar Rig 5 thru my Macbook Pro and Apogee Duet II for live stuff while I sort thru the Kemper to see if I can actually get a usable clean tone for a live setting. GR5's cleans fill the room up while Kemper's cleans sound like a squashed pancake that I have to bench press my guitar strings to get close to what I'm looking for. Sounds great for Hard Rock tho. Top Jimi's EVH, AFD and Caswell AFD profile packs are supreme for huge rock chords, but absolutely nothing in the "Kemper Clean Dept" that doesn't sound like a 5 Ross compressors are stacked on it.
If you are getting a good sound on headphones, then it points to something else in your chain.
SPDIF is not the utopian recording solution and I don't believe you necessarily need it. It could be multiple things and I suggest you start to rule things out before you splurge on a new interface.
Most interfaces perform a simple translation and I don;t believe ( although happy for someone to tell me something very different) you will hear "night and day" differences so don;t bin it just yet.
I would ( if you haven't already):
Check the set up of the interface, ensure your levels are balanced so that you are not clipping or sending a weak signal
Check your DAW for the same
Check a recording vs what you are hearing in monitoring ( can be different sometimes)
Check your montoring using the same headphones into your interface vs direct out of the KPA.
Listening through monitors is never going to give you the amp in the room sound so I'd leave those until last.
I think you've done much of this but just checking this systematic flow before you rush off to buy a new interface
I noticed there's a hi-z input and 2 mic-inputs on the soundcard...How do you connect the kemper to your soundcard?