There's no need for the 18". The Acme can handle all the lows you need. It will give bigger AND tighter lows than the Carvin 1x18 and much better top end.
If anything I'd consider a second B4 if you really need a ton of volume and you have an amp with enough juice to really move some air.
A fEarful 15/15/6 is another option worth exploring.
I think the added 15 or 18 is more of a rounding out the attack because of 18 reacting a bit slower than an added low end thing.
Im not honestly sure about the fEarful cabs. Im reading and passing on what i can but id like an idea of price, build time, and the general idea behind them.