You're welcome, velalv...
At the end of the day, it should be about what you (and your band members) like; not about brands or what others like. GK is a great brand and that is why lots of people use their amps. With the low B, though, it is a bit tricky. The low B string on an electric bass is around 31 Hz, and there are no speakers out there which can replicate that frequency even if it is in their specs. So, you tend to hear the low B more from its upper harmonics (B at 31 Hz, B at 62 Hz, the following F# up, B at 93 Hz, the following D# up, etc.). And a decent speaker (or a combination of speakers) should do the work in that case. And Bergantino is definitely one of those at the top. Even a single 12 should be fine; of course, not at kidney-stone removing level!
The problem with most basses with low B's is that their B strings are so floppy that they are not focused and they do not have proper sustain. Please do not get me wrong; your bass might have a great low B; but to my experience the problem with low B's is with the bass most of the time; not with the amplification or the speakers. That is why I would recommend you to rule that out before investing in a new amp and speaker.
Just my two cents... Good luck with the search...
Best,
Alper