it's not just SVS that applies EQ to their sealed subs. unless you want a sealed sub rolling off at 40-50hz, you NEED EQ boosts at the low end. so i would say, any and all sane subwoofer manufacturers would EQ boost their sealed subwoofers.
with regards to the rolloff difference, it's not that there is MORE of a limiter on the SB13, but there would be MORE of an EQ boost with the SB2000. i would think that if tested, the SB13 would still have more output at all the subsonic frequencies as compared to the SB2000. (larger woofer, larger enclosure and more power and all)
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OP, you should ask SVS/Ed Mullen about the excursion thingy though.
but without reading carefully the thread, aside from the visual excursion difference, what sound difference do you hear?
the SB13 is a lot "cleaner" (less distortion) than the SB2000, could it be what you are hearing "Extra" with the SB2000 is actually distortion instead of actual content?