Whether MAO selectivity is lost is a very interesting question. It's hard to determine in an individual due to variation in enzyme production, but also because most of the studies are done with elderly people.
Most resources say that MAO selectivity is lost somewhere around 15-20mg per day.
The manufacturers recently revised their opinion due to a few (less than 5, i believe) documented cases of tyramine induced hypertension in patients taking 10mg/day.
It's worth remembering that MAO-B levels rise as we age (indeed MAO-B is not present in early foetuses), meaning that increased dosing is possible...
I'm intending to start a course of 15mg/week soon (5mg sunday, tuesday, thursday) and see how that goes.
If no effect after two weeks I'll increase to 5mg, 4x a week
stz - I agree that dopamine receptor downregulation is likely. MAO-B does not trigger the homeostatic mechanisms as rapidly as, say, amphetamine (possibly because amphetamine causes a substantial increase in cortisol levels? not much is really known about this AFAIK), but anecdotal evidence (check out the selegiline section on
www.erowid.org ) suggests that for the first few days a stimulant effect manifests, which then subsides, implying downregulation.
stz - also, its not the MDMA metabolism thats the problem, its the metabolism of serotonin (absence of metabolism - serotonin syndrome), and the metabolism of noradrenaline (absence of metabolism - hypertensive crisis)