I am not sure you can blame smoking marijuana for amotivational syndrome when you were taking antidepressants because they can cause it without any weed at all. I have been on several antidepressants as they told me it would help control my pain and all it did was make me very tired and completely zoned out. I don't take any of that now and just smoke weed and I am just fine.
MAOIs are also very powerful and can make you very tired so there are too many variables here to know which one affected you. You are probably right mixing weed and those drugs is not good and this would make the drugs stronger but then it is the drugs making your lack of motivation more than just the weed by itself would.