Its simple really,
MAOIs mixed with damn near anything is an extremely BAD combo. MAOIs prevent your enzymes from breaking down chemicals in your body so the chemicals are still able to do their job, but they never stop. This basicaly means you will overdose on a normal dose of any recreational drug. Depending on the overdose limit, you can suffer minimal health effects to brain damage to death. Its not a good idea to mix this stuff together.
DXM works on several receptors but primarally on seratonin receptors. Ecstasy does likewise, as do SSRIs. Combining any of the above 3 is bad. If you do DXM and Ecstasy you will likely dump a massive quantity of seratonin with no way to uptake it back into your neurons. This means you will be overdosing on seratonin (seratonin sydrome) which can potentially kill you. This can happen on the following combos:
DXM+MAOI
DXM+SSRI
DXM+MDMA
MDMA+MAOI
and many other combos as well.
Most importantly, if you are taking any kind of prescription drugs like MAOIs or SSRIs just avoid drugs altogether especially MDMA / DXM. I mixed various combos of these at one point in my life and it is EXTREMELY DANGEROUS. Its also not any fun whatsoever and even if you make it through it ok (like me) you still don't know what the end result will be on your brain chemistry.
Lastly, for safety sake I personally recommend not mixing anything with DXM. DXM is an extremely powerful substance that isn't given a lot of respect. First of all, if you are hyper sensitive to it because you lack a specific liver enzyme, you will have a much different and probably terrifying experience than others would have on a similar dose. Secondly, it effects a huge number of pathways in the brain. It is not a 'simple' drug it is extremely complex and acts in many different ways especially on your seratonin system which in my experience, is the one you want to fuck up the least because it has so many key roles in the body and mind.