Hi Kaschko,
I have a Focusrite 6i6 and my set up has always been the Kemper into the 6i6 via S/PDIF with monitor speakers connected to the 6i6. I've never connected the Kemper direct to the speakers, as (like you) I use the speakers to listen to my PC audio via the 6i6 also - I therefore cannot comment re: whether there is any difference between the two set ups.
However, I'm confused re: your responses to BayouTexan. I think BT was suggesting that any comparison should be done at equal volume - my 6i6 has a volume control, so, in extremis, if it was turned all the way down and I compared the two set ups the Kemper direct to the speakers would sound better, simply because there would be no sound from the 6i6. The interface volume needs to be set accordingly to match the volume when going direct from the Kemper to the speakers.
Additionally, if you open the Focusrite control software, you should have the option to increase/decrease the S/PDIF level via the faders - independently of the Kemper. Again, I think BT was just suggesting you check that this is at 0 db and has not accidently been lowered. Although it probably is at 0 db if you've not accessed it before.
I'll keep an eye on this thread, as I'd be interested to know if I've been missing out by not going direct from Kemper to speakers.