I wanted to somewhat clarify my previous post regarding thermostat control of cooling fans. You need to be aware of the specific type of thermostat you need to control fan startup. I had mentioned using a line voltage heating thermostat. Actually that will only work correctly if the switch has double-throw contacts, known as SPDT (single pole double throw).
Many heating thermostats are only SPST (single throw single pole). What that means is the contact is open-on-rise. So the fan would run when the temperature is cold and then turn off when hot, which is the opposite of what you want. You need a close-on-rise contact, which is the way a cooling thermostat operates. If the thermostat is double pole then it can be used either way, and some line voltage thermostats are double pole.
The SPST heating style could be wired in conjunction with a pilot relay (as my OP mentioned). That would work, but adds a little more unnecessary complexity. Just use a close-on-rise contact is prefereable. Or if your preamp has any switched AC outlets on the back then you could do without a thermostat at all, it just starts up the fans when power is on. That's what I used. Or you could even wire a separate manually operated AC switch for just the fan circuit. So there are several various approaches. Additionally note that you only need one thermostat, not two. Just sense the temperature on one side of the rack and control the fans on both sides.
I spent some time in the Grainger catalog and found a good thermostat for this application. On page 4029 of catalog #401 (their newest) they show a Columbus Electric ETD-5S-6S, Grainger stock# 2E158 for $22.57 which is a good price. Rated 120-270VAC at 22 amps SPDT with only 4 degree differential (difference between turn-on vs. turn-off temperature).
Also they have a Robertshaw 803A, Grainger #4KFV5 for $21.26. This is an SPST but is for cooling applications so thats alright. Differential is +/- 6 degrees though, which means a larger swing between startup vs shutoff temperatures.
Theres also an attic fan thermostat SPST close-on-rise Garinger#3H JN6 for $36.50 but the differential is quite high at 10 degrees.
Also theres a remote bulb sensing unit close-on-rise SPST with +/- 3 differential, Grainger #2NNR5 for $55.35. If you dont have a lot of room in the cabinet or just want to keep the AC wiring out of there, then this would be a good one. Current capacity is very high too. I would use this one if it was for my own rig.
For fan speed control it depends upon the type of fan motor you choose. I prefer muffin fans due to a wide variety of choices and mounting flexibility, and low current consumption. As mentioned I use a variac for muffin fan speed control with good success at reasonable cost. Some variacs even come with line cord, on/off switch, and a load receptacle so you just plug right in and go. Variacs can be found inexpensively on fleaBay. You only need a smaller model because the load current of this application is minimal.
Other types of fan motors require a different type of speed control. PWM (pulse width) constant voltage motor control is commonplace. Some (a few) motor types can even be run on an SCR lamp dimmer, although dimmers can be noisy (RFI generation) so that might not be the best approach for an audio rig.
You have to know the type of fan motor and the proper control method for it.
Grainger has a number of different motor controls. The #4C331 is for shaded pole or split capacitor motors such as ceiling bathroom or kitchen fans. 120V at 3 amps for $24.39. This is not for impedance protected motors (muffin fans). I guess its a PWM type control; I dont know.
Theres also a #1AGV3 3-step low-med-high speed switch for $44.35 (only 1 amp current may or may not be enough for you?) for ceiling fan type motors. Also theres a #1AGU5 3 amp $25.85.
Spend some time on Grainger.com to learn more and call them with your questions. You can also email me; I may or may not have answers. Consumers cannot order directly from Grainger; youll have to go through a local electric motor shop or electrical supply house, etc. unless of course your firm has an account.
hope all this helps