If your power supply is non-Voltage stabilizer (i.e. the 13.8Volt is not electronically stabilized), you better off buying a Variac that controls the input voltage of the independent Power supply.
Variac is like a ?Rheostat?, but it will let you change the 110V.A.C that go into the Power supply.
Your Power Supply is 13.8V 10Amp that means it is 138Watts.
For safety, buy 150Watt Variac for 110V.A.C
If you have Voltage Stabilizer Power Supply, you have to vary the DC output.
You have to know the current that is used by each Fan, add them together, to get the total current that the fans use from the Power Supply.
Fans state the current, or the Wattage. To translate Wattage to current:
Current (Ampere)=Watts / 12Volts.
The resistance size of the Rheostat will depend on the amount of Voltage that you want to reduce.
Resistance = Max Change in Voltage / Total Current
I.E if all fans use together, 2A and you want to reduce the Voltage by 4 Volt.
The Rheostat (connected in serious to the Fans) should be 2 Ohms.
4volt / 2A = 2 Ohm.
The Wattage of the Rheostat = Total Current x Voltage
2A x 4Volt = 8 Watt + Safety margin.
The Rheostat should be 2 Ohms 12 Watt.
This is just a friendly advice. No guarantee