Moreover, I like the sound of the Stagi better - the Jackie is loud and strident, more like a "real" concertina I guess, but certainly not mellow sounding like my Lachenal.
I don't play concertina, but I have a good friend who's got a Dipper, and I would never ever describe it as "loud and strident". "Sweet and mellow", sure.
I thought the strident ones were less expensive concertinas with accordion reeds, and the stridency was considered bad?
Well, loud is not necessarily bad but by stridency I mean a sort of harshness. Of course, I am referring only to the Jackie, the sound of which has been characterized by some as being closer to a concertina-reeded instrument rather than the accordion-reeded instrument that it is. My Stagi is more mellow and much quieter (which, of course, some would regard as a fault). I have a Lachenal as well and it is as loud as the Jackie but sweet and mellow, like you say.
By the way, how are you coming along on the Weltmeister?