I will give my educated guess on the issue of taking MDMA while being treated with A/Ds. I'm limiting this to specific A/Ds, SSRIs and SSNRIs (Selective Serotonin Reuptake Inhibitors and Selective Serotonin and Norepinephrine Reuptake Inhibitors, respectively). Really, I'm just talking about A/Ds that inhibit serotonin reuptake, which is basically what an SSRI does; an SSNRI does the same as well (it differs by inhibiting the reuptake of norepinephrine as well). I'm not including SNRIs (Selective Norepinephrine Reuptake Inhibitors, not Serotonin and Norepinephrine Reuptake Inhibitors; the "S" is for "Selective" not "Serotonin"), TCAs, MAOIs, or any atypical A/Ds; I do not have enough information about these other classes of A/Ds affects when combined with MDMA to even attempt to guess as to their affects on the "Ecstasy high."
This part goes for someone who is on A/D therapy and has taken the A/D long enough to reach (basically) steady-state plasma concentrations. This is one part where I may be wrong. It usually takes around 3-5 days to reach steady-state plasma concentrations, but this could be more, I'm guessing very rarely less, depending on the specific A/D.
Despite that, A/D effects usually do not appear until 1-4 works and it can take up to 8 weeks (all rough numbers) before a person receives the maximum benefits from the A/D they are taking (again, depending on the specific A/D, as well as the person). The "educated guess" that will follow could be for one of the two listed scenarios, either when the A/D reaches steady-state plasma concentrations or when the A/D reaches is maximum effectiveness (or possibly anywhere in between).
Rule of Thumb: In the vast majority of people taking this combination, they will experience a much less "euphoric" experience from the MDMA (all aspects of the MDMA "high" will be significantly reduced). Also, in the vast majority, they may experience absolutely no effects from MDMA. Anyone who still can "roll," will most likely need a higher dose of MDMA to produce the same effects theygot when not taking an A/D.
Despite my Rule of Thumb, I'm sure there are some people who will have the exact same (or very similar) experience with MDMA when they are on A/D therapy as compared to previous experiences when they were not taking any A/Ds.
Conclusion: People on A/D therapy that take MDMA may have the same "roll" as they did prior to starting A/D therapy OR they may have no "roll" at all; people can also fall anywhere in between. It's my belief that many will not "roll" at all, some will "roll" but not as hard, and a very small minority will have the same (or very similar) "roll."
I believe taking any SSRI after the effects of MDMA have worn off, then continue to take that SSRI for several days following the MDMA usage will prevent some negative (possibly including neurotoxic) effects. This goes for someone who took MDMA but was NOT on A/D therapy and was just "post-loading" with an SSRI; it's my belief that an SSNRI would do the same.
Finally, it's my opinion that taking either or these types of A/Ds prior to taking MDMA (or "pre-loading") will lessen the effects of the "roll." I don't think this would normally cancel out the MDMA entirely because the A/D had only been taken once and that's no where long enough to get to the two scenarios I mentioned before, steady-state plasma concentrations or maximum effectiveness. I belief that "pre-loading" with an A/D in these classes will NEVER increase the effectiveness of the MDMA.
All of this is my educated guess and I do have some personal experience. I took MDMA roughly 10 times prior to starting any A/Ds (years before I required A/Ds) and I took MDMA on 3 occassions on 3 different A/Ds (Paxil, Celexa, and Cymbalta) and NEVER got any effects from the MDMA (friends taking the same pills but not on A/D therapy got their usual effects each time). Just my 2 cents...
EDIT: Fixed some (probably not all) grammatical errors.