I began this thread because I'm so happy with the sound of my Silver 6 speakers. I'd read that it had been replaced recently by the Silver 300 model, pointing that out in the OP.
However, it isn't that simple. As seen from several of the posts to this thread, the 6 went through several iterations, with letters, like RX-6 & RS-6, preceding the number. It seems the last Silver 6 was the final model of the 6 series. And it is relatively easy to see why they changed model designations.
The Silver 6 was the smallest floor-stander of its line, with the much larger 8 and 10 above it.
The new series two smallest floor-standing models are the Series 200 and 300. Each has some commonalities with the Silver 6. For instance, like the 6, the 200 is a two-and-a-half-way design, but rather than dual 6" woofers, employs dual 5.25" drivers. One may suppose this the reason for MA dropping the 6 as model number. The Silver 6 had a slightly larger cabinet.
The Series 300 uses dual 6" woofers, adding a mid-driver for its three-way design. So the guy who wrote the 300 replaced the 6 (in a review I read somewhere) was basing it strictly upon woofer size, it appears. I think it more accurate to say the 200 replaced the 6, considering its similar 2.5-way design, as well as both models' prices and entry positions in the Silver floor-standing line.
Of course, MA cites improvements. Now, I'm uncertain how much improvement the new tweeter screen makes sonically, but it sure does look cool as shirt. Other than the screen, they appear to be the same gold tweeter. Kind of wish they'd come up with that screen a model earlier (because I am, after all, shallow Hal), but since I leave the grills on all the time anyway, the visual doesn't make much difference.
I saw another review in which the writer opined that the three-way Silver 300 sounded better than the Silver 6 - not hugely better, but better. I should hope so. Haven't seen any comparison between the old 6 and the new 200, but have read good things about the small new one. Amazon still offers the Silver 6, new, in walnut, at just under a kilobuck/pair. An excellent buy if you've been considering purchase.
I'd like to think that thirty or forty years from now some vintage audio enthusiast will find this info of interest while deciding whether or not to restore mine.