I had migrated from SMBX to Mari0 when I had bought a Macintosh. That's because SMBX wasn't supported for Mac OS X.
With SMBX, you could use different GFX and SFX in different levels. It was until recently that I remembered this.
So that brings up the question: Possible to use different tile sets in different stages?
If this isn't possible, I apologize for making a useless topic.
EDIT: Problem Solved (For SE)
TurretBot wrote:And now for something completely different... AN EXPLOIT!
Make the first image graphics/smbtiles.png in your mappack, and the second one as both graphics/portaltiles.png and the normal tiles.png.
Congrats! Now you have like 4 tilesets worth of tileset to play with, and not even screwing up the SMB or Portal tilesets while you're at it.