On the discussion of the whole MD difference thing I find MDMA a bit like Acid. I eat acid a lot more than MDMA and personally feel there is a difference based on purity/synth. Likewise with MDMA, for ages there I was just getting standard 120mg pills, recently got some MDMA that I'd been told was special and 125mg along with few other things was the first time in a while I was tapping my feet in the taxi to a completely shit song on the radio. I always thought it since I ate the Q Dances, but people attributed that to MDA. It's a reason now why I don't take pills as often, I'd much rather wait and save my tolerance for something good to come along. I would say however, that for years I didn't notice a big difference in LSD, but with MDMA it was much easier to notice by the much stronger empathy (what I love MDMA for and why I stopped taking certain pills), loved up and the dosage could be slightly less ~25-50mg.
Having said that tolerance plays a massive factor too I think as does the setting and what you've eaten etc before - much like other drugs can be. I have eaten the same tabs from the same sheets of acid and had completely different trips, dirty/clean/funny/shit, bit similar with some MDMA pills so I definitely put that one down to setting or how I'm feeling upcomming to it. I don't think it's one particular answer, I think there's multiple factors to consider (purity/synth route/passion of chemist(?)/cleaning synth/setting/fatigue/diet/mood) - hope that doesn't come under synth discussion as that's as far as chemistry and synthesis go for me. Planning to buy some good brain pills though as I never looked into it properly before, need to get some money first though. Will probably consider trying what I thought was one of the more worse pills after doing so and see if I get the desired effects then, that way I would conclude it was due to tolerance. If it's semi there, you know it's tolerance and another factor then (personally I think it's synth routes and passion), as someone who has no real knowledge of chemistry I see synthesis like cooking when it comes to difficult to synthesize chemicals like LSD, were a little passion goes a long, long way. But even so, those batches of pills that don't quite do it for me are still completely fine, just since I'm mainly into tripping I can choose a bit more as a gram lasts me a few sessions if done right - unless it's a binge/festival. Also, if you take enough - whether you feel it's the top synth or just standard, it get's the hallucinations out regardless and that's what I'm after.