When you say “Polk”, you mean Massimo-Sound United (MSU), right?
Even if “Polk” wanted to make a bigger more expensive sub, MSU (who owns B&W, Classe, Marantz, Denon, Polk, DefTech) probably won’t let them - MSU already has a big expensive sub - the $6K B&W DB1D dual 12” subwoofer:
High-performance 12-in subwoofer
www.bowerswilkins.com