Some great suggestions already, so I'll tack in a slightly different direction.
Yes, the newer Portaflex heads and cabs can get you sorta, kinda in that tone range, and they can move more air (particularly if you get two cabs). I
Link Removed, and I did spend some time comparing them to my '66 B-15N. No, it's not the same, but I was surprised how close I was able to get them. I think that a PF-500 or PF-800 with one or two PF-115HE or PH-210HE's would probably serve you pretty well. Though admittedly, it's not going to sound exactly like a B-15N. An
Link Removed into one or two of the Portaflex cabs might be nice, too.
Another option would be to get a Jules Amps Monique pre (also
Link Removed) and run it into about anything (powered cab, power amp then cab, input jack on more powerful combo, etc). Jule also makes a powered version of Monique (800w class-D power section by Jim Demeter), though I have not heard it yet. This head into one or two of the Portaflex cabs would be nice, as well.
If you want more cabs and you want them to be faithful to the B-15 tone, check out
Vintage-Blue. Mark's cabs are to die for! He made the cabs for the Heritage B-15N.