The old M series (such as M31) were JBL copies of the D series. They were started when several manufacturers (including Fender and Shobud) asked for them after the D series had been discontinued. The number after the M seems to denote which manufacturer they were made for. I have a D-130 and an M (31?) in Marrs cabs. I play them together and can't hear any real difference. But I think there is a more recent M series with numbers such as you show, and these are a completely different speaker.
In general it seems to me that bass speakers just sound a little darker than guitar speakers of the same size, but they can still be good for steel, and some people even prefer them for being warmer. A bass cabinet is a completely different thing. They are usually much bigger and get more of the low-lows, possibly muddying up steel. And many of them today have tweeters (to capture finger and pick dynamics?). So for steel they can be at once too muddy and to screechy.[This message was edited by David Doggett on 19 January 2005 at 11:47 AM.]
