Don't forget that some fans don't do as well as others when using PWM to undervolt them. Panaflos, for instance, are unhappy with PWM, but will start and run just fine (and quietly) with resistance used to reduce voltage (rheostat for adjustability) - or with stacked diodes to reduce voltage. See
stacking diodes to reduce voltage.
The other thing to be aware of is that axial fans are wings. They have stall points. Normally stacking two fans together doesn't get you much but if you've undervolted to the point of being in stall, stacking a second identical fan (with plenum between to reduce inter-fan interference) may give you a better noise/airflow tradeoff than simply turning up the voltage.
It may not, too - one of those "try and see" things...