I doubt you are getting or hearing "low low."
I am betting that if you got an RTA and put a measurement mic on it, that most of what you like in the 18" loaded subs will be the higher order harmonics causing a "rumble" in the sound. The content you are hearing is prominent in the 80-100Hz and 160-200Hz bands.
There is VERY little difference in extension between an 18" and a 15" sub. The driver displacement to cabinet ratio tends to be more favorable for low end extension in a 15" cabinet than a 18" cabinet. The 18" woofers are rarely placed in a cabinet that is large enough to take advantage of the Fs of the woofer.
I use folded horn subs, which avoid the harmonic problem entirely. The folds act as a low pass filter and remove everything that is not "sub" content. The sound is VERY different, but once you go folded horn...you can't go back. All I hear with direct radiating subs is mid-bass mud.
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