I've got six of them, but I'm not a DB player. They have very nice low end extension. I love them on the drum set because of the low end extension and I use them for both vocals (while drumming) and drums because the mounting is so incredibly flexible without a lot of distance back to front.
Hypercardioid (there's a hot spot at 180 degrees off-axis, but it has a tight pattern).
Anyway, I love them. I can't think of why they wouldn't work well for double bass - or on an electric bass amp for that matter.
KO