I was only looking at the prog sweeps graphs which reflect the headroom across the subs BW. Using the 115dB sweep for each, where the JBL is already exceeding 10% THD at 40 Hz, the JBL gives 9% THD at 60 Hz and the PB Ultra gives 5%. Of course, the JBL will play louder above 40 Hz because that's what it's designed to do, but it's claimed to be tuned to 25 Hz, so I look at the system when looking at a single frequency. This is part of the problem. When the JBL is pushed to it's maximum output numbers, it reeks of harmonics above and below tune, giving over 300% THD below 20 Hz, which unquestionably changes subjective perception of its SQ.
Filtering the sub as part of a 2-way will change the subs headroom numbers dramatically, so the OP is doing the right thing. I just have 2 issues: 1) 60 Hz is 60 Hz. It's far less troublesome and cheaper to just use the proper drivers in a smaller package and avoid a crossover altogether and 2) I would choose a higher cross point for the reasons I've mentioned and looking at the subs actual FR.