I just can't point out what it is. The difference is increasing further with added effects as i said. The sound stage is way better on the Kemper. Its like you can hear a wider spectrum. I don't know what causes the differences.
I didn't really hear any really difference in the effects it was just that the levels were different and also that the Kemper had a generally warmer or fuller sound.
As for sending signal back through the Kemper to monitor. Yes you can do it. I'm not sure if it works over SPDIF but you can send a mix from your interface to the Return Input and Alternative Input then mix these with the guitar sound using the Aux levels in the Output section. You control the level of the mix input from the Aux In>Headphone volume .
There is a video about how to do it on the Kemper website.