I think that the PB is a much better option over the SB12+ in terms of low end output, flexibility and above all that, SQ. They just revamped this sub with a whole new driver and should be a top notch product. Much like the Ultra13, when 3rd party measurements come about, I'll put money up that it will measure incredibly well...
The SB12's are great little performers for music and HT, but don't have the sheer grunt that a ported box tuned low will offer you... I have the SB12 for my son, and its a powerful little monster, but starts rolling off in the mid 20hz range.... perfect for music, but for HT doesn't offer that same power in the rumble zone the PB12 will....
