Nothing wrong with wanting more, just figure out what you want more of.
Empires roll of rather sharply below 20 hz so you won't get much more output below that no matter how many you have. Or to put it another way, if more sub 20hz output is your goal, look in another direction. If not, then another Empire is the most cost effective way to go.
A submersive would be a step up for sure, but at significant cost. I would try to determine based on your listening material and output goals whether the cost would be worth it (it may be). After that exercise, figure out your desired end state, and work up to it in steps if you need to.