I have seen speakers above a monitor affect a monitor in a Midway (e.g. pacman) or Stern (e.g. SuperCobra) cabinet. However a good degauss with a professional wand usually removes it.
At least, try to remove the speaker for a couple days and degauss again to rule it out entirely.
Have you tried to degauss the metal frame and mounting bolts? You will need to remove the bezel and get up against the frame to ensure you degauss it.
Finally, it is possible if the monitor or cabinet was dropped or otherwise roughly handled, the shadow mask may have gotten warped inside the tube. I've seen it happen on new consumer TV's shipped across the country before, so it does happen, though they are usually a bit more sensitive than 25 year old tubes)
Also you may have convergence or other issues that require reconverging, or using metal tabs or specifically oriented magnets to deflect the yoke in the corner