Well I'm being greedy, as I'd like to see parameters best suited to a LLT. In regards to the low Bl, I started examining parameters of drivers that model extremely well in a LLT and those that have a less flat FR
above 20hz. Aside from Le, Bl was a big indicator - drivers with really high Bl don't model as flat above 20hz in a ported enclosure. I then came upon this article, which seemed to give that finding some credit:
http://www.aes.org/sections/uk/meetings/0403.html
As for a higher Qts, again, this is from looking at drivers that model extremely well in a LLT - Dayton Reference HiFi 12 and 15, Avalanche 18, Dayton Titanic 15, Tumult 15 and 18, etc. - they all seem to have a Qts around or above 4.00.
Now the RLp15 works in a LLT, as is proven by the many happy owners, but it's not ideally suited to a LLT, as it has a moderately high Fs, low Vas, high Bl, and low Qts - moderately. It would seem it could definitely be improved for this application. My desired parameters, aside from a low Fs, are almost the opposite of what one wants for a sealed sub. Again, it's just me being greedy I guess
The LMS 4000 does a great job of hitting all these points (almost too well in fact), but unfortunately the sensitivity is pretty poor. If that 4100 stays about the same but has better sensitivity, it would make for the ideal LLT driver.