One way to potentiate methadone if you are talking about antidepressants, is to take it with fluvoxamine, or possibly fluoxetine or sertraline. These SSRI's are all potent CYP450 enzyme inhibitors, inhibiting several of the isozymes responsible for methadone metabolism. I remember a few years ago when I re-entered a MM program, I got my doctor to switch me to fluvoxamine because I knew it would boost my methadone levels by 25 to 50 percent, maybe more. I never got higher than a 45mg dose as I was constantly on the nod and ended up on the psych ward because I could not take care of myself anymore. Of course my brain was quite fried at the time as I was at the end of a year long crack binge, had lost 60 pounds (from 195 lbs down to 135 lbs), and was watching my marriage go up in flames around me, so there were other things, but I know that when I had not been on fluvoxamine in the past when I had been in the program, I could tolerate much higher doses, and now I feel strung out many times on a daily dose of 114mg.
Grapefruit juice, Tagament, fluvoxamine, each of these drugs does not really potentiate the activity of opioids, they increase the levels of the opiates. Yes, that means you are getting a stronger effect with any given dose, but that is because you are basically taken a higher dose. Usually when pharmacologists talk about potentiate they are talking about something acting by a unique pathway, causing any given serum level of opiate (or whatever agonist is being studied) to have an increased effect, or a broader spectrum of activity. Things that would be in this category would be things such as alcohol, benzodiazepines, possibly antihistamines, cannabis, and so on.
At least this has always been my understanding of the material