Careful there bro. I went through that same phase about 3 years ago. The speed even seemed to bring back the magic. Not long after I'd been mixing the two, I noticed floaters in my vision, and developed this real annoying thing called Scheerers phenonenon, or blue field entoptic phenomenon. This is where you see what looks like fireflies all over in your field of vision, especially in the bright light. It is most annoying. These are associated with HPPD (but not HPPD in their own right), and I started to experience really annoying after images, all the time, but only after combining the two.
Also, I bet you $100 bucks, that if you try just plain MDMA a couple weeks after a MDMA/meth roll, you'll just feel fucked up, no real high at all. The only way for you to experience that same buzz is to mix the two again, every time.
Your body/mind somehow adjusts to the extra power/intensity in the speed rolls, and as soon as your body senses the MDMA it assumes the speed is coming too, and makes all sorts of adjustments, so in other words, you'll begin to experience a sort of comedown as soon as the MDMA starts to come on. Which really sucks, expecially if you are expecting a high.
This all probably sounds pretty weird, but meth mixed in with MDMA is seriously bad ( in my view, though for a while I was truly a believer), and for MDMA alone, it will kill the magic, very fast.
My personal opinion is that the two become slightly neurotoxic when combined, for reasons unknown.
I strongly suggest you stick with either one or the other at any one time, never combine them.
This is just my experience, and if I could go back I would never mix the two, no buzz is worth the problems I just mentioned, and if you develop these symptoms, I promise you you'll be reminded of your drug expriences every day for the rest of your life.