Here's a link to a forum with a big discussion on the B200.http://www.audiocircle.com/circles/viewtopic.php?t=19253&highlight=visaton+b200
I don't know how Visaton measures them.
As far as getting a driver that will cover the range from, say, 80-10,000 Hz, consider the ones I use, vintage Electro-Voice LS 12's or LS-8's. On a 2 1/2 foot x 3 foot baffle, these both play down to 80 Hz and up to 10 kHz, and do so reasonably flat. Both respond well to standard fullrange driver tweeks, like adding phase plugs. Both are available for under $100 a pair, sometimes more like $50. The LS-12's are supposed to be around 94-95 dB sensitivity, but seem even more. The LS-8's are a dB or two less sensitive. The 12's ahve a bit more mid-bass energy because of bigger cone and higher Qts, so can use a slightly smaller baffle for about the same bass response. Although I haven't tried them, I suspect that the SP-12's and 12TRXB's would also do good on OB's.
BTW, the LS-12's come with both AlNiCo and ceramic magnets. The ceramic ones weigh around a pound more. Ceramic ones have a black gasket on the front and a black nameplate on the back with silver lettering. The AlNiCo ones have a blue gasket on the front and a silver plate on the back. At least the ones I have are like that.
Dave