basic amp tolerance is actually caused by dopamine transports adapting to not let amphetamines in as easily (originally it sees amphetamines the same as dopamine)
anyway, here's my theory on the matter (I actually enjoy amps more with 5-htp btw)- serotonin and dopamine create a yin yang relationship in the brain, more of one means less of the other. If you add 5-htp to the mix, the dopamine isn't going to be as prevalent because serotonin is taking up some of the space. Amps and cocaine like drugs give you your intense good feelings by flooding your brain with dopamine, 5-htp is gonna make the amps work harder to do that. On the other hand tho, amps can create anxiety normally because dopamine pushes serotonin out, and lowered serotonin causes an anxious state, 5-htp will prevent this
also, my thoughts on the reason why sometimes it causes e not to work is because mdma relies on your brain's ability to pick it up like serotonin, if your stores are massively full already then not very much mdma is gonna get in. That said, 5-htp can also make mdma potentiated at lower doses, likely because if the mdma does get in, you have more serotonin for it to play with
a lot of this is theory based on things I've read (how 5-htp, serotonin, dopamine, mdma & amphetamines work individually) so if anyone has any factual info to touch up on it with then feel free
edit: btw, 5-htp builds up serotonin in your brain, once you feel good you can probably do to forget about it until you start feeling shitty again- taking it regularly tends to overdo it (for me at least)
same reason why coke kills a roll if i remember.