I've been playing around with dual GRS 12SW4's in BP4 in HR. I tried the BP6's, but the graphs were ugly.What does the excursion of this design look like in the 20-30hz range?
The response of this design looks very similar to the Quad GRS 10" 6th order series tuned bandpass sub I built a little while back. One of the main issues with that design was that the peak in the response (~105-110hz on mine) caused an amplification of the driver 3rd and 4th order harmonics in the 25-35hz range which just happened to coincide with the excursion maximum where such distortion was most prevalent.
The JBL's are likely cleaner but I'd watch for the same kind of rise in distortion in the 20-30hz range.
Edit: looked at my measurements again and it was just the 3rd order harmonics that got boosted.