Lab 12 is not good much past 150-200HZ I try not to use mine over 80Hz.
If you wanted to make an acceptable passive xover it would have to be custom designed and optimzed. You will probably have better luck with an active Xover.
65 Watts?? A little under powered for the Lab 12.
I'm guessing this is all just for a home stereo.
I can't make any value judgement on the Isobaric, as I know what Isobaric is but that doesn't tell me anything else specific about components and design parameters.
It being flat but not a tight sounding subwoofer doesn't really mean anything to me. As I don't know what your bassis for comparison, but makes me fell that you may be falling for the legend of the Subwoofer with good Transient response, which is a contradiction.
Anyhow Sounds like fun, I suspect it will need to be quite large.
Antone-