Yes, and it suc..., uhmm, did not do it well.
JBL subsequently used HL91 at 800 Hz in several products, but ultimately dubbed it a "1200 Hz" horn. Check later literature, and you'll see it was progressively derated.
At 800 Hz, even, the longer HL92 is preferred, as used in L300 and monitor products.
In 1976, JBL was still calling HL91 "500 Hz," but HL92, introduced around that time, "800 Hz."
http://www.lansingheritage.org/image...comp/page9.jpg
By 1979, HL91 was a "1200 Hz" horn:
http://www.lansingheritage.org/image...omp/page10.jpg
Horns generally have a reputation for sounding, well, "horny," and JBL's early [mis]application of HL91 played a role in that.
I'm saying using an active crossover will allow you to make an empirical determination of the optimum crossover frequency for your particular combination of components....