I also use Delta's PWM at my rigs.

As the other said, the possible thing is those fans have refurbished (adding old motor into the new case), even the case was broken. As far as I know, Delta fans case was a solid and strong enough. Next, maybe you also need to add PWM controlling board (with slight modifications to the wiring scheme) to manage all of your running fans (for adjusting fan speed needed at several conditions).