B/C or C#/D is also good, but what IS the point of putting any reeds in the bass end?
Al, I think you need to get out a bit more, or at least listen to some of the Irish box players who make very effective use of their bass ends.
Might I suggest, for starters, Jackie Daly on C#/D and Billy McComiskey or Martin Quinn on B/C, and Joe Derrane on D/C#? I think you'll be pleasantly surprised.
True, 8 or 12 push-pull basses won't cover many of the keys that you can theoretically play in on semitone boxes. But you can do a fair bit in the core keys used in Irish trad. Lots of players don't use them much or at all, but that's not because they cannot be of any use.
To Bauld // - I think the answer all depends on the style of music you want to be able to play. I'm very happy with C#D for Irish trad. For a wider variety of music, BCC# is clearly considerably more flexible than C#D. But generally you are looking at a considerably heavier, bulkier and more expensive instrument. If you play DG then I think BCC# is going to present you with a much steeper learning curve than C#D.