The problem with running 'Windows Sound' through your KA6, is that sometimes Windows automatically changes the sample rate to 48KHz, which changes the KA6 sample rate. That would then mean that your Kemper, which is locked at 44.1KHz, won't work, because the KA6 is now set to 48KHz.
If you go to Windows Control Panel, then find Sound, locate your default playback device with the green check mark, which should show the KA6 - highlight the KA6 and click properties, then click the Advanced tab (shown in the picture below). If yours is showing 48KHz, and you can hear Spotify through your monitors currently, then you've located the problem why the Kemper can't be heard, the KA6 has upped it's sample rate to 48KHz. Windows Sounds must also be locked at 44.1KHz.
To fix this, change the Windows Sound dropdown box that says 48KHz, and select 44.1KHz 24-bit stereo. That should allow you to hear Windows Sound and your Kemper together.
If you only require playback function from your PC, you could also try connecting the monitors directly to the Kemper, then use the outputs from the KA6, in to the AUX in the back of the Kemper.