6/12 live question about Quads

[Rottersod]

never thought I'd have a question about how to play my Quads but I'm starting to get back into limit from NL and this hand just came up. I don't remember the exact details of the starting amounts but this will suffice:

8 players. 6/12

MP: $400
CO: $500+
Button (Hero): $400

UTG calls, UTG+1 calls, MP (aggressive) calls, MP1 folds, CO (aggressive) calls, Hero (I've been playing TAG and haven't shown anyone any big bluffs or unusual plays) has K [img]/images/graemlins/spade.gif[/img] K [img]/images/graemlins/heart.gif[/img] raises, SB folds, BB folds, UTG folds, UTG1 calls, MP calls, MP1 folds, CO calls.

Flop: K [img]/images/graemlins/club.gif[/img] 8 [img]/images/graemlins/diamond.gif[/img] 7 [img]/images/graemlins/spade.gif[/img] UTG1 checks, MP checks, CO checks, Hero bets, UTG1 folds, MP raises, CO calls, Hero raises, MP calls, CO calls.

Turn: K [img]/images/graemlins/diamond.gif[/img] MP checks, CO checks, Hero bets (I hope one of them hit a FH), MP raises, CO calls (huh?).

What is my move? Do I re-raise and figure both of them filled up and they'll keep paying me or do I flat call? I can't think of any river card that would slow them down except an A if they put me on AA or AK. I figure I just keep raising but want to get some opinions. This was very puzzling turn betting.