At the barn I ride at, we have a 13 or so year old QH who was usually ridden in a snaffle-mouth curb with four inch shanks. He HATED it. Every time you tried to turn, he just blew up on you. Tossed his head, ringed his tail, braced... We tried him in a snaffle again, but he hated it just as much.
Someone recommended me try a classic Mikmar bit. It puts pressure on all areas (Tongue, chin, poll, nose, etc.), and Cody seemed to appreciate it. He acted lovely with it.
He likes it a lot more than his other bits.
Actually, we put this curb back in last week to see if it was really the bit that helped, and he turned right back into old Cody. Definitely hates that curb; loves the Mikmar.