Today I got a Cougar Vortex V14S (1,000 rpm) which only turned downto ~720 rpm. Also two Antec Truequiet 140 (800 rpm)
which have a high / low switch. One did ~450 @ L and ~750 @ H, the other ~420 @ L and ~720 @ H. But when undervolting
with the fan controller both couldn't go below ~630 and ~600 rpm. Puzzling, if anyone has an idea why, please tell me.
Different resistance for the fans themselves? As these fans are rated at a lower RPM it's fare to assume that they draw less current and therefore have a higher resistance themselves. Take a look at the current ratings of each fan to confirm the resistance via V=IR.
When you put a fan controller inline all do you is put another resistor in series. This causes a voltage drop across the fan as some voltage is acting across the resistor. The fractional difference in the voltage depends upon the resistance of the fan itself. As the resistance of the fan itself is higher, the effect of adding more resistance through your fan controller is less and therefore the volatge across the fan is greater than it is for the faster spinning, lower resistance fans that you have tested before.
Amazingly some resistor cables actually sell as a '5V adaptor cable'. Absolutely nonsensical unless they've been designed for one specific fan!
I would suggest you try running your fans on 3.3V, 5V, 6V, 7V or 8.7V by physical wiring instead. Inline resistors are not efficient or give much of a dependable result.