Here is the Horn Board.
The gap between the Horn Board and the L-Pad board is roughly 21-7/8" minus 15-3/8" minus 6-5/16" = 3/16"
The grille measures 21-7/8" wide x 20-3/4" high (including cloth thickness on the edges) and there is a gap between the top of the grille and the bottom of the Horn/L-Pad boards similar to the gap between the Horn and L-Pad boards. 1/8" to 3/16". It would appear the tolerances are 1/16" as opposed to 1/32" but that might not be true.
The tuning is accomplished using two 4-1/8" ID ports each having an 8-1/4" duct.
It should be noted that this tuning has been termed as being "aggressive" for Home Hi-Fi use by several JBL engineers.
Their recommendation has been to tune down into the 28 Hz to 30 Hz range.
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