I have my own personal opinions on what you have said and the reasons behind it.
But first a few points:
MDMA is NEVER produced from MDA.
Those recipes are describing ways in which to get REAGENTS from over the counter sources. Once someone finds the precursor, often without connections, obtaining common industrial chemicals without attention is very difficult. You still need that precursor. Anyone arriving at MDMA without it is using very sophisticated techniques and reagents.
Most pills seem to contain MDXX - meaning an MDMA like substance or perhaps a combination - e.g. MDMA and MDA.
The number of pills that are MDXX has certainly increased over the last 4 years and if you assume what happens in the UK and the NEtherlands is also true here, then dosage has gone up also.
There are a number of "chemical" influences on how a pill will be:
The identity of psychoactive chemicals contained in the pill.
The quantity or dose of those chemicals contained in the pill.
The types and levels of the various impurities left over from the reaction process used to manufacture those psychoactive chemicals.
It just so happens that the majority of pills that will test as MDXX to a tester are real MDMA and it is the only psychoactive chemical they contain. The dose can vary widely but it will generally be from 50-150mg.
The impurity content ALWAYS changes, as no two chemical reactions are ever the same. Impurities however may not cause any appreciable difference in the subjective effects. They could of course increase the pills damage to some of your internal organs such as your liver, but you are unlikely to detect anything like that. Of course on occasion there may be impurities that have marked effects on how the pills feels - we will never really know however.
Tolerance is a HUGE issue. It would not surprise me if the two pills you said had a good effect on you recently were actually MDA - this is NOT cross-tolerant with MDMA, so for people who have used MDMA for too long, a MDA pill once in a while can be a nice surprise.
HOWEVER in my opinion, bar the obvious issue of taking a break, there is one major factor that people have overlooked. For new users to MDMA the feeling is so unique and so special, that provided they are comfortable, they could do almost anything at all and have the most amazing time of their lives. For users who have used frequently and over a long time, that ability to just pop a pill and be in another world is lost. You notice how the physical effects are still very present, but the mental empathy and euphoria is gone. On occasion though, it seems to come back. WHY?
Yes you may have got a stronger pill on that occasion.
But it is far more likely that it was the set and setting you placed yourself in that transported an all too familiar experience into something that approached how you felt when you first began taking MDMA.
It is true for everyone. I am tolerant enough now to have a strong pill and completely switch its effects off. This is especially true as the effects wane slightly as because you know the drug so well, you know what is happening. It is hard to not just forget about it and coast back to baseline in the bliss that you should be.
However on other occasions I can have an ordinary MDMA pill and have the time of my life - why; simply because of the time and place I chose to take it, the company I chose to take it with, and my attitude and motivation at the time of taking it.
For first timers, "E" is easy - it will take you places you never dreamed of, without you even having to try.
For the pensioners out there on the scene "E" becomes very different - you control it, and only when you use it in the most positive of settings, with the most positive of reasons, and all the time intent on having a great time, with or without the drug, will that long lost euphoria return.
MDMA experiences have to grow with their user - it can always be special, it just wont be in the same way that it was when it took an innocent young man or woman into a place they had not been before.