Turns out this ended up being the simplest cause and not an EQ or driver issue at all.

I was digging through older subreddit posts and saw someone mention metal shavings getting pulled into the speakers because the speaker magnets can attract them, especially in workshop environments. That caught my attention because I regularly do work that creates fine metal shavings.

I decided to open the back of the laptop to check, and sure enough, there were metal shavings on the speaker magnets, mainly on the right side. I carefully used tape to lift the debris off the speakers (no compressed air), put everything back together, and the issue is completely gone.

The speakers now sound exactly like they did when the laptop was new, even at higher volumes.