obviously I meant similar bracing or lack thereof.
So you think the 2 panels will be enough for that size box? This is by far the largest box I have ever built and the first to use real bracing.
Are all walls 3/4" material? If so, I'd say you probably want at least three braces along the 55" dimension. With two braces, you still have unsupported lengths of at least 18". Go to three braces, and that drops to just under 14". I'd use four braces myself, that gets your span down to 11". My box has 48" wide panels, and I used four braces across that width - though two additional near the center.
If your walls are 1" or thicker, you might be able to get away with two braces. Also playing a factor will be the strength and excursion of the woofer.
All that said, I don't know if there are any hard and fast rules for bracing. I don't recall reading such in any of the usually referenced loudspeaker design literature. Most DIYers tend to err on the side of overbuilding, rather than underbuilding. Most people tend to make braces from the same material as the enclosure, ie 3/4" MDF, which they join to all four sides of the cabinet, then cut a big circular hole for air passage. There's nothing wrong with this method, except that it adds significant weight (could be a benefit depending on your point of view) and takes a lot of extra material, most of which will be cut out into big MDF discs and removed. I prefer to use dimensional lumber (2x4) for bracing, unless I just happen to be making a divided cabinet where I want to seal off one side from the other. 2x4s are easy to add at any time even after the box is full constructed (but before veneer or paint are applied). Other people are fans of bracing with wooden dowels of 3/4" to 1.5" thickness, and that can also work well.