I've used four of these woofers in a few different setups.. they aren't tuned super low (can't really get 20hz out of them at all), but they have a powerful motor and lots of xmax and perform very well in properly tuned cabinets.I built a electric bass cab for my friend. 5.3 cuft, tuned at 36hz. really nice sounding, f3 of 50zh and f10 of 30hz (only a 1db hump at 100hz), it really sounded good, and was within excursion limits at full power.
I also have one each in a pair of 8cu ft sealed cabinets, which I'm going to tune to probably 33hz to give an f3 of 45hz and an f10 of 25hz. Only 6db down at 30hz.
These woofers sound very nice. They go really high too. and are very smooth (small 1 or 2db humps at 4k) until they drop 10db at about 4.8khz and then there are breakup modes after that. Truly 99 - 100 db efficient though.
They perform very nicely. I still have the JBL pdf for them if you would like it.