For the 2nd part, all mics need to be powered correctly in order to work right, specifically when dealing with higher dB levels. The mics you're using need to be powered with either a battery box or a preamp, regular plug-in power won't be suitable. Always use the preamp to supply power to the mics. Without being properly powered, they will distort.
Likewise, always make sure you're using a battery that works and isn't dying. That would lower the voltage and not supply enough power, which also creates distortion, and that's one of the first things people here would suggest to double check.
The ST-20a preamp I'm not familiar with, but I believe it probably has fixed gain settings. It's possible it could have overloaded your recorder if you were using a +20 or +30 setting, so you'll want to use a +0 or +10 setting instead. Also, always use the "line-in" input, not the "mic-in". Mic-in adds gain unnecessarily. And just to be safe, avoid all settings on the recorders such as auto-gain control, etc, make sure they're set to off. Doing all of the above should result in a clean recording, unless perhaps the preamp is broken, which I doubt is the case.