Welcome to the PJB club. In my experience, once you tried it, it's hard to like the tone of anything else. Re. your questions, the C8 cab is 4ohms. It obviously packs more of a punch than a C4, but check the BG400 can run an ext cab with that impedance. I've an idea it might be 8 minimum, which limits you to a C4. Two C4s (the BG is effectively a C4 with built in amp) is pretty pokey, within reason. Whether they will do the job for you depends how much of an animal your drummer is. I have 3 C4s, but my head will run into 2 ohms. PJB cabs are quite inefficient - they take some driving to give of their best - so even if your BG400 can run a 4ohm ext cab, you may need more power to drive a C8 properly. The PJB Piranha drivers have ceramic magnets, the Neos have neodymium and are lighter. I find the Piranhas have a bit more oomph, but are not quite as smooth/sweet as the Neos.