That spot appears to me to be the kind that a degaussing tool will not readily fix. Try rotating the entire machine 45 degrees while it is on and see if that fixes it.
If the spot goes away when you rotate it, then you are just going to have to leave it in it's new position, or return it and wait. I have 4 Donkey Kong machines, and they are all sensitive to being rotated in one position or another. I will give you an example:
I had my DK Cabaret against the wall for 6 months, no problems, bright beautiful picture. I moved it across the room, to the other side, and I had a weird colorless, dull spot in the lower left quadrant. I tried degaussing, and it made it slightly better, but not 100%. I left it as is. After a few weeks of regular use, the spot went away.