Oh..and yes..the Kemper is the only modeller which does not add anything to a Marshall type of profile.It is by far the "least forgiving" of all modellers because it reacts exactly like the (well) profiled amp.If you play it (Marshall circuit)on very low volume it sounds ugly.I always have said that the fractal stuff sounds like "on steroids" which is maybe a good thing celebrating ourselves as guitar heroes at very low volumes deep in the night but it does not help "in the real world".Everything "amp like" needs some volume.I can tell you that some time ago a lot of very famous producers/players used the ADA MP1 because it added a "nice compression" for "easier shred" to their beloved Marshall type of sound.Even at low volumes.Actually it was very famous for this.
i can totally relate to that. I didn’t realise that I actually LIKE some Marshalls until I got my KPA and p,ayed some Plexi style profiles. I have hated every single time I have had to use a real Marshall because they “sound ugly” as you put it. The reason is almost certainly that I was never able to get those beasts running hard enough to hit their sweet spot and start really working. As a result I just thought all Marshalls were shit. I now know that when driven properly some of them can sound glorious but I only found this out thanks to the KPA and some great profiles.
As for the OPs original post there are two possible problems.
1 - noise gate on the input settings is killing the sound. Just turn it off then increase very gradually until it cuts unwanted noise wihout affecting tone and feel.
2 - playing quietly or even silently (IEM) is just a totally different experience to playing a real valve amp at stage volumes. That’s just physics not a flaw of the KPA.