Well I will definitely be bracing. There are a number of large areas the vibrate. In fact, when I first had them outside a plane flow over (not low) and I could feel the cabinet walls vibrate. Not good!
The side walls under the horn flare vibrate a lot as do the lower front panels where the ports are. Those could certainly use bracing. The lower back panel also vibrates a bit.
In other words, any large unbraced area vibrates.
I have some 2x2 lumber that has been drying in the attic for about 20 years. I'll use that for horizontal bracing. Will try to use the 3/5th rule when placing the braces. I'll look for hot spots after that as GM suggests. Might add mass to those.
I'll let you know how it goes!
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