the main argument against delay or reverb before the pre-amp is that the delay will be distorted if you're using more than a little gain. for cleaner tones it'll work fine, usually. or if you've got a distortion effect in the chain as well, put it before the delay or reverb, and then you're adding space to the distorted sound, rather than distorting the reverb trail, which would sound unnatural and, frankly, s***e.