Thread: What is missing ?
Enjoy the spare space for a while and maybe you'll find out which modules you actually need by using what you've got ;-)
Maybe something to pan stereo and mono signals would be an advantage so you could ditch one of the STMix's.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.