I've owned MG-Is and the original 2.7is; I had the 2.7is for five years before moving to a home that couldn't accommodate them and taking a different approach. The 2.7i in particular was an incredible speaker, very lifelike. I eventually settled on my grail speaker, Quad ESL63s, which I've had for 15 years.
We recently bought my son a system as a graduation present, and he chose the 1.7i. In that process we heard the whole line up to 3.7i, but this was before the new 2.7i came out, and before the x introduction. They're all great, in very similar ways. As you move up, you get better, fuller, more extended bass, although the fundamental character remains the same. As many have observed over the years, as you move up you also experience larger, if not better, images.
When I bought my 2.7s, the next one up was the 3.6i, and I listened extensively to both. I ultimately chose the 2.7s because I felt the bass was very close to the 3.6, but the overall presentation was more coherent, the speakers disappeared better and there was just music in front of me. That may be something they fixed in the 3.7i, and that may be even better in the 3.7x. That said, Jim Winey, the founder, did say the 2.7i was the best speaker they'd ever made, and at the time he said it I agreed.