You are probably right about the cut being bass management!
Where did you put your crossover?
With the 5-1/4" woofer, I could see it bottoming out if you play it loud and low.
How far away from the speakers are you (as this directly influences how loud you would play them)?
I think
@PENG found that crossing the LS50 at 100 or 110Hz worked best for him, I don't know if he can add much or not, he probably remembered that you are using LS50's (I didn't)! Maybe he has at one point found a similar limit.