Hi cbgood,
I have owned MG I, MG II, MG IIi, and MMG's. The main difference between these speakers are the size of the mylar panels and baffles (the larger baffles cut down on the destructive interference from the back wave of the dipole drivers) increasing the bass response. All of these use the same mylar with different sections to supply the bass & highs.
I think (but not sure) the 1.6 is made this way. The 3.6 has a true ribbon tweeter. The guy that bought my 1's, bought the 3.6 for the front. It has a true ribbon tweeter, which gives it extended high frequency response, but some think with a sacrifice of some coherence (whatever that means). I liked the 3.6, but with my old and abused ears did not hear a great improvement from the planar magnetic drivers of the originals.
We drove to Ohio (the closest dealer to PGH) to audition the 3.6 and my buddy bought them on the spot. After they arrived, he was not happy with the sound until he upgraded his amp for the front channel to a big MAC 2300. Originally he was using a Pioneer high end HT receiver with "125-150 watts" per channel.
As a side note, I still have a set of IIi's with upgraded xover parts that I like a lot. A buddy of mine redid the drivers, a doable task I have now done successfully myself. The hardest part is soldering the aluminum wires to the terminals after completing the rewiring of the "voice coils". The kits are available from Magnapan for a reasonable price. I still love the idea and the technology of the original speakers, which I believe was adapted brilliantly by infinity for their emit tweeters.
If you are looking to get into that sound on the cheap, you might want to contact Hal, he has a few sets to sell and is asking a very reasonable price(IMO).