What would it take to balance a stereo power amp ?

I am in a band and own the P.A. system that we use . In the interest of eliminating hum and noise I have decided to run it balanced . The main FOH system is tri-amped with an electronic crossover going into three seperate stereo power amps. Two of them have balanced inputs but one of them does not . The "unbalanced" amp is an older model that has given me years of trusty service and I don't want to replace it if unnecessary to do so . What would it take to balance it ?
Obviously , it would have to be an inexpensive retrofit or it wouldn't be worth the cost or aggravation . Any help would be greatly appreciated .