I traded out both AXY4's (I know it's a five-string, by I have the 'Classic' neck taper where the 4-string shell is appropriate) for FatBoys on the BigRedBass.
The 'Boys do sound more 'MusicMan-ish'. I describe the difference like this:
The AXY's sound like very, very good studio monitors: Letter-perfect representation, but that monitor-ish 'flat' EQ feel.
In comparison for me, the FatBoys sound more like high-end stereo speakers: A very good representation, but with some dips and peaks in the right place, more 'meat on the bones'. May be possibly just a bit louder, but a very small difference.
Although you can't go wrong with either one, I would not go back to AXY's, they were always just a bit pristine for me.
And why Alembic does NOT build a replacement Activator with a FatBoy in a MusicMan shell is beyond me. One of these in a StingRay with a proper Alembic harness would be extremely formidable, and I really like OEM MM pickups.
J o e y