I agree, methadone is potent as fuck, strong enough for you not to feel heroin when you are o medium doses like 60 or 80 mg a day; 120 or 150 or more mg/ day is an enormous amount of drug, if you don't find it enough and you want more by potentation (specially with benzos/ booze) or by rising dosages, it will lead you far from any safe harbour.
After many years on mdone I grew to understand that the only way to fight eternal constipation and to get other opioids to work again was tapering to a medium-low dose. Quitting mdone is hell but tapering to such doses is a realistic target.
As for potentation everything is already said in this and many other threads: antiacids, grapefruit, antihistaminics, certain antidepresants.. are on the soft side of potentiators, booze and pregabalin potentiate it much more and finally you have benzos or barbs if you really want to push things to really dangerous levels. It goes without saying that I strongly advise you against that behaviour.