I've tried grapefruit juice and turmeric (which supposedly are potentiators for kratom), but I didn't notice any difference. However, I take mirtazepine at night and notice that it somewhat does intensify the experience, though not by any measure that I find profound. I would think diphenhydramine, hydroxyzine and promethazine would probably potentiate it, or at least have some synergy, but that might over sedate some people. I personally don't think it's possible for me to get into a nod on kratom, because it was almost impossible for me to nod on IV heroin, even when I fixed after consuming dangerous amounts of clonazepam. At a certain point opiates lose their magic and there's nothing you can do about that (other then quit and reset your tolerance), but be happy with the effects that you do get from them. I would really like to try mixing kratom with some kava kava, but even that I'd be skeptical about as I have a benzo habit. I've also tried taking megadosages of kratom, and while I get a nice anxiolytic effect from it, I can only think of one time when kratom actually made me feel sedated.
That's the one thing I don't get about it, I hear a lot of people say that higher dosages cause sedation to the point where it's uncomfortable, but for me, it seems that past a certain dose, I get the opposite effect and feel way to stimulated, to the point where I'll start shaking. I've tried this with a few different strains, but to no avail.