Funny thing is in my experience, with a single SM57 , you usually obtain the opposite result: a hars and thin sound.
SM57 is a standar in micked amps because it add the texture or the grain you have listened in legendary recordings, but... usually sound too thin, with a poor bottom and a little hurting mid frecuency.
So, why you obtain a dull sound?
Probably you have to push the mic to the center of the speaker. The most harsh sound came from the mic pointing straight to the cap. Try a bit closer to the center of the speaker. Take a phone with the light on or a flashlight and observe carefuly the position of your speakers behind the cloth. And use the position guidelines we talk about. Just that!
Some facts:
-You should be able to get a harshy sound of a sm57 in any case (In fact, usually it's what you want to avoid in this mic)
-If you use low volumes on valve amps, it sound like if you put a blanket over the amp. But if you push the volume extremely hard, you can obtain too much compresion
-Kemper work very very well profiling all kind of Marshall sounds. It should nail it
Some tips:
-If a speaker isn't sounding good and don't obtain results changing the mic pos... use another speaker (it's common to have a speaker damaged or even dead) or even other sm57 mic (it's a sturdy mic, but I have 3 and 1 it's almost dead and have different response)
-If you don't have studio conditions (where you are able to hear what is coming from the mics without hearing the direct amp) you must have to improve or develope a way to do it: use a DAW or the X32 to record some riffs, them stop and listen what you have recorded. If you don't like the result move the mics to a new position and repeat the process until you find a usable balanced spot.
-To get a sound from an amp with mics it's all about what mics are receiving. The "amp in the room" sound doesn't matter. Only what you are recording matters. So if you need treble or presence in the recording try to give it in the amp too. Even when the amount of highs in the room could be disgusting.
-You can use and record several mics at the same time in parallel tracks to compare between mics or blend them to obtain the sound you want. But remember profiling process with a blend of mics need to be combined in a mono bus of the mixer to send it to the Kemper. And this part is always better with analog mixers (avoiding 2 digital conversions)
I hope now you can get the picture and make a new profile. But, i'm telling you... after this we will want to hear those profiles !!