I plan to make two Jacks although the plans show you how to make it a 210 or in my case a 212. For bass there is no real advantage between 2 110s and a 210, but the former is more flexible.
It's big because it's a semi-horn loaded design. So the box is a big part of maximizing SPL. Even in a normal reflex cab, adding volume to the box can improve the low end SPL dramatically.
My next project after finishing my ESB-4 will is to do two Jack 12 Lites. They're still relatively big for 112s, but they use thin plywood with aircraft bracing to make them light. I plan to make them swith able so that they can be used either for bass or PA....and so that I have the lightest possible cab that can still keep up with a drummer ;-)
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