With how light weight and powerful modern bass amps are is there any reason not to have enough amplifier to cover the most extreme situation you'll find yourself in? For me modular is the way to go. I've got an Aguilar TH350 I mostly use with one Bag End single 15 but can add another matching cab if I need louder. If I were playing louder music than I am I'd go with a TH500. Everyone's situation is different but I could do any gig I'd ever say yes to up to a sports arena with that amp into 2x15's. I could also use that head with a single 8" cab on a quiet restaurant gig. Powerful and modular, yes.