I know this post is a little old . . . haven't been on the BB in a long time either but I'm stuck at home sick on a Saturday night :roll:
I recently had a MM Axiom output transformer installed in my Stiletto Ace head. It is not a perfect fit - the mounts needed to be flipped and the red lead wire needs to be extended by 5" or so. No mods to the amp necessary, but a tech will probably charge you about $100 to install it. I notified Paul @ Mercury and they should be including a longer lead wire on new ones.
anyways, it does not change the nature of the preamp. it's still bright and cutting. it does, if you can believe this, add even more harmonics and enhances the midrange detail. it's like icing on the cake. I was bored and had $$ to burn - it's worth the money but not a necessity. just something nice to add if you've owned a Stiletto for awhile and want it to sound fresh.