(Warning: I am NOT a doctor, so take what I say with a grain of salt)
Zyban is sustained-release bupropion HCL, an antidepressant. Most anti-depressants are either SSRI (Selective Seratonin Reuptake Inhibitors) or MAOI's (MonoAmine Oxidase Inhibitors)
Taking MDMA with an SSRI will most likely result in feeling little or nothing from X, as the SSRI will block the MDMA from entering the 5HT cells. Taking MDMA with a MAOI is VERY dangerous and can result in seratonin syndrome, and possibly death as a result.
Bupropion is a mild SSRI, and also a weak inhibitor in the reuptake of norepinephrine and dopamine. Based solely on this, I would assume it would weaken your your roll slightly, HOWEVER bupropion also carries a very small risk of inducing seizures, theres no way to tell if MDMA will increase this risk.
IMO, I don't think its ever worth mixing MDMA with any other psycoactive compound, so I wouldn't risk it.