There isnt a lot known about DMAA itself in the first place. I doubt it would have much of a potentiating effect seeing as DMAA is next to useless, but I can't comment on that because I have never combined the two and wouldn't. The stuff is banned now anyway, I would just chuck it - whenever I used it I helped me focus a little bit but it felt wierd and the comedown wasn't worth the "high" that it gave.
In terms of combining Piperazines with MDMA, its be known to cause horrible comedowns. I personally don't think that the reason for this is that they both compete for the same enzyme because a large amount of the MDMA is actually excreted unchanged. If you lack the CYP2D6 enzyme, which some people do, or it is blocked (due to piperazines or whatever agent is competiting with MDMA), it won't so much make your roll potentiated or roll harder, it will most likely just increases its duration a bit if anything.
The reasoning behind that is MDMA tolerance builds extremely fast. Say you bomb 100mg, you come up, you peak, your peak is slowly dropping off then you start comming down... Your blood MDMA levels will still be almost as high as what they were at the "peak", say you drop another 100mg, your blood levels rise even further and are probably for examples sake 80% higher than what they were at your first peak, yet you will not have a peak that is as good as the first one even though blood levels are almost double, again it is due to rapid tolerance.
Essentially blocking the metabolism of MDMA to achieve a better roll is the best part of pointless because of rapid tolerance build up. There are other ways to potentiate MDMA, but if you have your head on straight you would know not to go down that route as chances are you will wind up dead or left with permanent severe damage.
