Recently acquired a used OP-1. Speakers were working fine for a while, but now they sound tinny – although it really only seems like an issue with the drum sounds. Tried to do a scan to see if there are any effects or functions enabled that would lead to that. It’s kind of a tinny, filter-y, almost like chiptune-y quality to the sound. Hoping I don’t need a replacement speaker…
Have you done a factory reset?
just to double-check that you haven’t turned on one of the effects on drum tracks or master?
I thought the speaker on my second hand OP-1 was damaged too but the issue turned out to be that the little speaker was being over-driven by the drive setting in the mixer.
Press mixer  next to  under the display then  and wind back DRIVE until the speaker stops farting out. However you may want to increase the drive again when not using the built in speaker; it’s just a reference tool really and can’t realistically handle the output OP-1 can deliver. Red Means Recording shows the mixer in use on his tape tutorial here; it still leaves me with questions though. - https://youtu.be/Z_8apw716HA?t=910