No, I plug into my Marshall with my LP it sounds excellent. I don't have to wait months for it to sound good. If someone tries a £1450 piece of gear and it doesn't sound satisfactory to them, in my experience it doesn't get better. I'm happy to know what people are doing, to get their Kempers sounding the way they want them.
You may plug into an expensive Marshall and not like it. Someone telling you to be patient is a load of drivel, unless they suggest how you can get the sound your looking for with the gear your using.
So what are you actually doing with the Kemper to make it sound better? Apart from patience, what's your actual technique?
i never tried this Seinheiser HD 75 headphones
but they are 70 ohms and i am currently using the Koss PRO4AA Titanium that is 250 Ohms
also, if you record many minutes of your playing with different profiles (alternating then) and listen to that you may start to hear that in a different way
i did not mean to be lack of information in saying that you need time to get used to the tone and your ears will distinguish it, it is not a tube amp, if you want to have the amp in the room feeling you may try like increasing the level
disabling the Cab to check how your system react, try pushing kemper into your real cabinet or some pre amp to have more headroom (sorry if this word is wrong i did not recall now)
i know it is not like your tube amp so do like people said, profile your own marshall and you will have a very close sound (assuming that you will use the same cabinet)
on your kemper profile the cabinet character is a parameter that you may try to tweak a bit...
my post was serious and may seems metaphysic or nielistic but your ear will get used and you will notice different sounds...
i like my kemper more in 12 inches monitors and with a good sub woofer
when i got it i also have that thinking to sell it or not, but now i could not even thinking on selling it, rather sell my tube amp than my kemper, i just keep it to have that vintage feeling and look on my studio...