If we're being intellectually honest - most acts have to team up with other big name acts just to fill a bill.
Last year I went to see Aerosmith and KISS.... when did you ever think you'd have to pair those two up just to fill a place? Last year those guys had a GREAT opening act too ... Saliva.... good, good band.
Look at Ozfest.... some of the biggest names in modern rock (and I think they are more rock last year than this years batch of nu-metal).... but last year Ozfest was Marilyn Manson, Korn, Disturbed, and Linkin Park - which are some of the biggest names in modern rock and heavy metal (and all acts I am really into right now

)....
All others are in bars. I don't blame Styx for having to team up with other bands because they likely would have to do it to tour in bigger places anyway... However I could think of other acts I'd like to see them with:
Kansas
Night Ranger (I would go to the show and stay for both acts if they toured with Night Ranger despite not caring for Carot Styx/Stynx).
Matchbox 20
No Doubt
Tesla
U2 (Styx would have to open I'm afraid)
The Who