Mike, opinion #1
If you wanna do it....
It's actually pretty easy to measure the output of your preamp, given that you have or know someone with an o'scope. A cheap one is fine for audio. You can take an old RCA cable, split it in the middle and strip the wires so that you can attach the probe to the hot and the ground lead to the shield of one channel. Make sure they don't touch! Then, play a test CD with a 1khz test tone. Set the scope up to measure the output signal of the head unit, turn up the output until the sine wave starts to flatten out top and bottom (clip) then back it off until it is at the max undistorted output. It is pretty obvious. THen, using the gradient on the scope (or a true RMS voltmeter) measure the size of the AC signal. (.707 * the peak voltage) (1/2 of the signal) is the RMS voltage output before clipping. Once you've found the max undistorted output of your head unit, do the same for the output of the preamp.
I would say that the bigger problem you would have would be matching a high output preamp to multiple amps from different manufacturers, they each are probably designed to handle a slightly different range of preamp input voltage levels. You will have to back down the preamp output until the amp with the lowest voltage handling capability does not clip at it's minimum sensitivity setting, then set the other amps to match. Good reason to use the same amps or ones from the same manufacturer. The "voltage" gradients on the input level pots of most amps are usually not accurate.
I used a six channel preamp so that I have three independent pairs for my three different amps.
Clipping LEDs usually don't flash until the signal is heavily clipped, you will hear it in the speakers before you see the clip light flashing.
Measuring the output signal and clipping of the amps requires BIG dummy loads that will get hot, but that is the only accurate way to see if your amps are clipping. You measure it exactly the same way. If you have a two channel scope, you can have both the input sine wave from the RCA and output sine across the dummy load. On the scope, you can put them right on top of each other, to look for any distortion (output not shaped like input in some way)
Still, use your ears, but if you measure it you have a reference instead of just tweaking until it sounds OK.