The question I have is how long the valbazen seemed to work. If they came back a few days later, yeah, they are resistant to the valbazen and time to try something new. If it was months, the valbazen worked. They got reinfected from new insects, and I would complete the last dose of valbazen and then watch and see.
If you start them on Equimax tomorrow, make sure that you throw away the eggs for 2 week following the treatment, re-dose in 2 weeks, and do a second 2 week egg withdrawal. And continue to try to eliminate the host insects. No drug will protect your chickens from re-infection after the first couple of weeks.
Good luck with whatever you decide and let us know how it went.