21's might be the answer if you want more output overall. Have you thought about adding a VBSS or two tuned at 31Hz or 40Hz? The Ultimax is a little light on mid-bass and the PA460 used in the VBSS excels at it. The two actually complement each other well. It would be a lot more inexpensive to add a couple of VBSS's to your Ultimax's than it would to go to 21's.
7.4.4 Theater Room: JVC-RS500, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic UB420, Apple TV 4K, JBL Studio 530’s, Dual Driver VBSS
3.1 Living Room: Samsung 64” F8500 Plasma, Anthem MRX 300, Dynaudio Audience 52’s, Dynaudio Audience 122C, NHT SubOne, Roku Express (2019)