Okay, I realize I don't have perfect or even close to perfect form, but I'm having an issue. When I'm playing my guitar (epiphonelpstandard) without an amp, or even if im playing just an acoustic and guitarcenter or something, I can pretty much always get harmonics to go. When I plug into my spider15w (**** i know), i can still pretty much get harmonics to go with 2-3 tries. When i plug into my bugera 333-212, it seems to be a little less free giving with the harmonics. Possibly because of my settings? But no matter what, i cant seem to get a good sounding and long sustained harmonic out of the low E string. I cannot grasp how metal/heavy metal bands throw their low Es. I apply vibrato and do it but it rarely works unless i physically stop after i pick and touch it with my thumb, but still the sustain sucks. ??
Pinch harmonics are just one of those techniques you have to do a lot to get good. Practice...a lot. If you can get pinch harmonics to work on an acoustic guitar, you're doing something right.

I doubt it's your settings, but mess around with them and see if that works, too.
It's all down to your technique, I remember when I was learning, I thought I'd never get a real pinch harmonic, and then one day I just found the spot.

Really dig in and try moving your picking hand in different places on the string.
weird, its usually easier for me on the lower strings than on the higher ones, except for my b string, doesnt like the pinch harmonics as much for some reason.
Im the opposite kinda.. The low string can get kinda squealy by accident more often than the high ones, but its easier to get a purposeful squeal on higher strings than lower for me..
Also, could it be my pickups as for getting a poor signal or something? I know my technique isnt perfect but still, my squeals seem more responsive without an amp lmao.
could be, its alot harder to do a pinch harmonic without the gain and alot of treble, and your pickups could obviously have an affect on that...its weird that its easier for you to do without an amp than with one...