Yeah this would line up with the isomer theory as well. I don't know how well researched MDMA's isomers are, but any drug will have a left and right isomer. In amphetamine for example, the psychoactive properties come entirely from the dextro isomer, which is why dexamphetamine is more potent than mixed amphetamine salts (Adderall) which contain both D-amph and L-amph. If you have just L-amph you'd only get the physical effects (fast heart rate etc) and no high at all. Just the D-amph and you get less of the physical effects but the full psychoactive effect.
MDMA is a substituted amphetamine so it's not exactly crazy to theorise it works in a similar way. If one isomer is responsible for all or most of the desirable psychoactive properties, and a bad synth produces too much of the shitty isomer, you will get MehDMA. A balanced synth may well be the more traditional "old school" stuff while a synth leaning towards only the most powerful isomer would have the most potency but not necessarily the most magic - although if MDMA is similar enough to regular amphetamine, in theory the active isomer would be cleaner and have fewer side effects, but again I have no idea if this is the case and am just speculating.
This might also explain why people are saying you could take higher doses of the older stuff and it'd cause less toxicity. If this is indeed true, it makes sense that you had balanced isomers, whereas the new stuff might be produced so you only get the strong psychoactive isomer, making it more potent per milligram similarly to Dexedrine vs. Adderall.
I'd have to do some research into how much is even known about MDMA's isomers to know how much merit this theory has, but isomers make a big difference for many many drugs so it is very plausible especially when you're talking about black market lab synths.
