To the best of my knowledge the priority of a meta box is limited to: Low, Default and High. Which is unlike admin menus that let you specify a value and put it anywhere. I'd try "High."
You can change the order of all your meta boxes by dragging and dropping so where ever High puts the subtitle you can drag it to be beneath the Title. The only disadvantage is that you'd have to do it for every user. There are plugins like Adminize that let you specify the order of meta boxes for all user accounts which would overcome this problem.
Create Subtitle Meta box: www.spencerfinnell.com/2010/07/05/wordpress-custom-write-panels/
Adminize Plugin: wordpress.org/extend/plugins/adminimize/
*I don't have a high enough rank to post links so you'll have to copy and paste