Merged Profiles?

They are profiles with a cab, so you treat them as such, for example killing the cab profile when using a (real) guitar cab.

It's not generally true they sound muffled, there might be some issue with your settings or procedures. Do they sound different from studio profiles to you? How do you listen to them?

If you make your own Merged profile from a Direct profile of the amp and merge the speaker from the Studio profile (with miked up cabinet) of the same amp with identical settings.
its very important that you check that ALL amp settings including gain are the same for the two profiles Before you merged the speaker from the studio profile.
If not the kemper cant make a right merged profile. I have had the same problem as you with dull sounding merged profile, and the problem was not identical amp settings.
My problem was that I checked all amp setting but forgot to check the gain setting.
The Kemper did not give the same gain setting for Direct amp profile and the studio profile of the amp.
As I have a Kemper Power Head, I have tried severeall profiles after the merge prosess to record the miked up cabinet connected to Speaker output (monitor output),
and Left main out with merged speaker profile on two separate tracks in Cubase Pro.
When I blind folded A/B the two tracks they sounded indentical. So the merge prosess Do work.
but be sure to check all the amp parameters before you merge the cabinet...

Yes, untick cab if you are using guitarcabinet and not a full range monitor (FRFR)

And unless you are making your own profiles from scratch, don't hit the Merge button!