I'm pretty sure this was all a gimmick concocted by music industry suits to try and compete with the more well known California Sound phenomenon. I think the California Sound has some merit because it feels distinct and forms something of a consistent style, though of course there's a lot of variation and experimentation there. With the Bosstown sound... I don't see anything common between The Ultimate Spinach and The Beacon Street Union, they're just completely different, is a band like Ill Wind part of that too because they were from Massachusetts? I don't think so, to me it feels like the "Boston Sound" is so specific and not a thing that the Spinach might as well be the only members (they were the best too in my opinion. Check out their first two albums, skip the third.)
(also the image was shamelessly stolen from their discogs page, unsure of the source but I thought it was interesting enough that I wrote this post)