Wattage doesn’t directly translate to dB, nor does it dictate how well something can drive speakers. Speakers are quoted to have a typical resistance, but that can vary drastically. Your amp is a class D amp, and doesn’t provide enough current for the LS50s during complex and demanding sequences. You mentioned that with content with very complex treble can make the LS50s sound a bit muddy - this sounds, to me, like your amp is clipping. If it cannot enough current to move the transducers quickly enough during a complex rhythm, the amp will clip, which is perceived by us as distortion
This is why having a 150W power amplifier for a pair of LS50s doesn’t mean it’ll be louder, but rather that the movement of transducers carry more energy and inertia when playing a track.
As a near field monitor, however, you typically wouldn’t notice a difference at lower volumes
I don’t use speakers personally, and have opted for a pair of P9s with a V90 HPA.