I've decided a few changes in music are in order:
Doppel-Gunnoid no longer has two battles and may either get one unique theme or share a theme with the other bosses. Personally I'd rather go with the former because it's part of the final boss. I'm hoping to add two (or one) tracks for the Mechano Arena boss fights, though the final bosses still keep their themes.
Mechano Arena X will not be getting unique boss themes because the bosses already have unique battle themes (aside from The Taboo who all have one theme and another boss who shares Doppel Gunnoid's theme (s)).