THIS IS NO BS! Way back when Chicago hosted the CES show I had a conversation with Mr. Paisley who was one of those responsible for the design of the original M1. We talked candidly about the M1 (which I owned at the time) and the soon to be released M1Si. He was very straight forward answering my questions. He explained that when they came up with the idea of the M1 it was meant to be a flagship type product which they planned to sell in very limited numbers ("a few hundred per year"). To their surprise, demand for M1's was huge and the current production could not keep up. The tweeter for example, was a custom unit, hand tweeked with tight tolerances. The crossover was also hand tweeked to each tweeter to get the best sound. When demand grew, things would have to change so the production could keep pace. The real reason for the tweeter upgrades was to speed production. The new tweeter had tolerances that allowed it to be used with no hand tweeking involved-and the "new" crossover changes were implemented to work with the new tweeter parameters. I never heard the M1Si in a side by side comparison with my M1's so I cannot give you a comment on any specific sonic differences. I can say however that I never felt the need to upgrade to the M1Si even after hearing them several times. I ultimately moved on to 2-way stand mounted speakers and do not miss the Mirage bi-polar sound. My M1's, by the way, were low serial numbered and were bi-wireable.