The M30's and M40.1 use fabric dome tweeters whereas the other Harbeth models use metal dome tweeters. As the "M" speakers are designed to be drop in replacements for the BBC LS5/9 and BBC LS5/8, I'm wondering if the tweeter choice was dictated by the BBC spec. If not, would it be possible for you to give us your thoughts on the decision process involved in determining which tweeter to use for domestic and professional speakers.