whether this is just a property of the 5 inch drivers in the monitors vs 8 inch in the amp, or if there's more to it.
When you run direct into a "flat response" system, whether that's headphones, studio monitors, a cheap plastic FRFR (e.g. headrush), or a stadium PA, you're just taking the signal from a mic on a cab in another room (or sometimes with bass the direct output of an amp or preamp pedal), and making it louder. Any of these is a much different experience than cranking a live guitar cab or bass cab in the room with you because no modeler can magically transform flat response speakers into a cranked 412 guitar cab or 810 bass cab for example. If you want that experience there's no way to accurately get it besides disabling the cab block in the Fractal and running it into a power amp and guitar/bass cab.
Think about being in an actual studio. The live guitar cab is cranked up in the live room. You could either stand in the live room with the cab, or you could stand in the control room. Even if it's a multi-million dollar control room, the two experiences will be dramatically different. But for me, what matters is what makes it into the DAW (or to the PA system in a live environment), so the experience of standing in the live room with the actual cab is irrelevant.
So the good news is that you can play back any
recorded guitar or bass tone you've ever heard through your studio monitors to give you a good target to aim for with the modeler, because you're using the same monitoring paradigm for both and the live cab has been eliminated from the equation. If a recording of guitar/bass sounds good through those KRK monitors in your room then you know it's possible to achieve very similar tones with some combination of the Fractal, other outboard gear, and plugins.
buying different monitors, getting a dedicated FRFR cab
Both of these options will suffer the same issue as above. Worth noting that the KRKs are very much entry level so you can dramatically improve the quality and accuracy of what you're hearing with better speakers, and more importantly, acoustic treatment. Good acoustic treatment for bass frequencies is not cheap however, and DIY'ing 3" panels filled with building insulation will do nothing for the low end.
So you'll have to decide if you want to forget the "studio" paradigm and just plug into some live guitar/bass cabs and play without any regard to the accuracy of what you're hearing, or do you want to invest in improving your home studio for the long term. Nothing wrong with either approach unless you're trying to replicate one experience with the other.