How can you change subtitle filters?

I use K-Lite with MPC to watch anime a lot, but newer subs tend to use libass instead of XYSubFilter, which can make certain things appear wrong if the player isn't using libass as the filter.

I might just be being blind, but I didn't see an option anywhere in the settings? Can anyone help? Thanks!

You are confusing things. Those videos use "Advanced Substation Alpha" (= ASS) subtitles. Libass, XySubFilter, VSFilter, etcetera all are capable of handling that type of subtitles.

I recommend using the internal subtitle renderer of MPC-HC.
Options > Playback > Output

That's what I'm using, let me try to explain my problem better I guess.

There's some typesetting (fancy sign subtitles) in a video I watch, it was made using libass, it doesn't display correctly under anything else.

When I view this video in MPC-HC with he K-Lite Codec pack, it displays incorrectly. In say, MPV or VLC, it displays correctly.

It was not created with libass. That is a subtitle decoder, not a creator. They just tested with libass only when creating the subtitle and forgot about compatibility with other renderers.

There is currently no option to use libass. But there may be one in the future for the internal renderer.