Halfway down the page below we see some measures of the 123A. Looks like you may have been picking up baffle diffraction artifacts, compare the smooth response on the curved baffle to the nasty jaggeds from the terrible diffraction of the stock box.
Also, this is a beast of a woofer and deserves a beast of a box, the L100 are painfully undersized. The driver is made with subwoofer optimized thiele smalles, but happens to avoid the most serious misbehavior up until the kHz range. It's much more efficient than most subs, with a .5 Qts (or so) and a 25Hz Fs with a 90-91dB ref. efficiency. One could easily get a lot of output with 50W in a 6 ft. sealed box.