It is my opinion that most folks who believe they lacked top-end ran them ported.
I owned & ran a Brahma 15" in a sealed enclosure in my vehicle for some time and had no such observation of the top-end. I even did a test pairing it with a tweeter and a passive crossover and mid-range was also fairly impressive considering it was a sub-woofer.
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It's the same reason people think our X, Z v.4, etc lack top-end... the lack of parameter shift over stroke :
http://www.diysubwoofers.org/misc/dumax/details/adire_audio_brahma_12.pdf
KMS vs. X is very similar to our new platform drivers except it is not over as wide of a stroke range (naturally... as they used an 8" spider and a standard frame) -- but in it's range it is MEGA flat.
BL vs. X is ALSO very linear thanks to XBL technology -- this further reduces QTS shift over stroke -- even beyond our stuff.
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Now go check my article about the new platform sound :
http://www.stevemeadedesigns.com/board/topic/187407-sundown-x-and-z-v4-sound-difference-visualized/