If your cleans sound good, it is because they have a lot of dynamic range.
Higher gain sounds have NO dynamic range by their nature of being a clipped/overly compressed waveform.
The freqs that tell your brain it sounds good are Highs( 2.5k and up) and Lows. These also tell your brain "this sound is close to me". Most studio monitors have a very flat high freq response. And most guitar profiles have no bass response. So high gain ends up being almost ALL MIDS with no dynamics, which tell your brain "this sound is far away from me" and "it is dull".
The more gain you have the more you are relying on the lows to have some dynamic range. The mids and highs will have no dynamic range. If you record high gain guitar you will see the volume is constant until you start chugging. Then it gets twice as loud. Because most gain requires removal of lows before the gain stage. Then the lows are boosted back up.
I have fought with your problem for years when I got my Kemper. My answer was different speakers.
My Kemper sounded great thru cheap speakers and sounded flat and lifeless thru my monitors. Cheap speakers are all highs and lows, sound great. Monitors are very flat, sound bad.
I ended up replacing the tweeters in my monitors and now the Kemper shines. If I boosted the highs the Kemper became shrill and harsh. New tweeters was the only way to liven them up to get around the natural boring response of the original tweeters.