I've tried agmatine and memantine both (still have giant bags of both that are never touched) not for MMT tapering but during my time with experimenting potentiating/lowering tolerance to kratom. Neither were particularly successful as far as I could tell, which wasn't very. And I hated the effects of both. Yuck. I will say that the agmatine did kill any euphoria I may have achieved from kratom. So it actually ended up doing more harm than good.