Pre Amplifier FX and Post Amplifier FX differ mainly in the way the sound being effected is usually processed. For example, Distortion or Overdrive FX are usually applied pre (before the) amplifier so only the sound of the instrument is being distorted and not the sound of the instrument along with the sound of the amplifier. Reverb or Delay, on the other hand, is usually inserted post (after the) amplifier so the effects themselves won’t be processed by any Distortion or Modulation effects.
However, there really is no rule book when it comes to creating presets. Some of the most classic sounds in music history have been created using sounds or FX that didn’t "go together". Try to experiment with different techniques and sounds. Who knows? By inserting that Spring Reverb before the amp after a nice Overdrive effect, you may just find the next classic sound.