I think serotonin syndrome is the most likely thing to happen (convulsions and other unpleasant things). XTC causses elevated serotonin levels, so if you stay beneath the serotonin-syndrome level, you would get some XTC-like ffects.
I once took a tryptophan capsule with a little syrian rue and indeed noticed very mild XTC-like effects, but these types of experiments are kind of dangerous. The neurotransmitter wich in freebase form would be most interesting maybe, is PEA, phenylethylamine. PEA is sometimes sold as some kind of suplement, so widely available.