Welcome to the club!
I’m a little confused. Your title says your amp sounds muffled but your last sentence says you’re playing out of studio monitors. If you were playing out of an amp, then cabs are supposed to be turned off as your real amp/cab serves that purpose.
If you’re playing out of something that is supposed to be FRFR, like studio monitors, then the cab sims are meant to be turned on. If that’s the case and the sound is muffled, make sure the high cut on the cab or an eq block isn’t turned way down.