How should I slow these fans?

[ColdMiser]

I am using 2 Panaflo 120 H1A's in a push-pull config around my rad. I want to slow them down b/c they are way overkill at full speed. Here's the specs on the fans:

2500 RPM,
103 CFM,
.6 Amps,
5.5 Watts,

I have been doing research and have found many options, including rheostats, voltage regulators, 7-volt trick, potentiometers, etc.

I want to use a method that:
1)Is healthy for my power supply (I'm kinda leery of the 7-volt trick)
2)Will not result in the slowing device getting very very hot.
3)Can have both fans under the 1 device.

I have looked at rheos, I just don't know size to get, etc. Please help!

[Rooster]

one nice thing about rheostats is that in the middle of summer when it gets hot you can turn them up

[The Doors]

Or use the 7v trick, and during summer mod the fans Molex connectors for 12v :D