I've wondered if the quality of the 5HTP has anything to do with it, I used to consume 1g 5HTP daily and was on strong tricyclics at the time too, no indications of serotonin syndrome - that only came from smoking a blend that had Rue in it while on the meds (foolish and a seriously bad time)
Years later after being off the meds I purchased some cheap 5HTP which made me feel really unwell at low dosage, strange.