KLowe, you used a preamp to raise microphone signals to line level, then you fed those line level signals to the line inputs of a recorder. It helps to think of your setup as a series of consecutive "stages" that the signals go through. You apparently set the recorder's level controls properly so that its meters never reached 0 dB (full scale). That rules out one possible way of causing an overload, which is good, but there were still at least four other possible stages at which it could have occurred:
[1] The sound levels at your microphones could have been greater than the microphones were capable of handling.
[2] The voltage put out by your microphones could have been greater than the inputs to your preamp were capable of handling.
[3] Preamps are basically "voltage multipliers." If you send 0.01 Volt into a preamp that you've set for 40 dB gain (which is a factor of 100 in voltage, assuming a high-impedance load), you're asking the preamp to put out 1 Volt at its outputs. That's usually fine, but then if you send in 0.2 Volts instead, suddenly you're asking the preamp to put out 20 Volts, which may well be more than it can put out. In effect you're overdriving the output stage of the preamp if you do this.
[4] The voltage put out by your preamp could have been greater than the inputs to your recorder were capable of handling.
Not knowing the characteristics of your particular equipment, I can't say which of these is most likely. But the general concept is to locate the problem by considering each successive stage in the setup.
Keep in mind that many preamps don't indicate input-stage overload (item [2] on my little list); if they have "clip" indicators, they generally are attached only to the output stage (item [3] on the list). So item [2] can be a real problem in live recording because you can't see it happen while it's happening--it just happens.
I only use preamps whose inputs I know can handle the full output voltage that my microphones are capable of putting out; then I never have to worry about it. But if I did have this problem, then resistive attenuators ("pads") at the preamp inputs would be the first thing I'd want to try. They're cheap, they're easy to use, they're reliable, and if you use pads that have just enough attenuation to avoid overload, they don't make the recording any noisier than it has to be.
--best regards