How to replace the speaker grill cloth???

I am not aware of the specific set up of the JBL grills, but I have successfully used 3M spray adhesive to attach cloth to speakers with metal frame grills, and those with the grill cloth over a wooden facing. If you search around here and AA's Speaker Asylum, there are posts about how some have tacked the cloth down first to help align it properly, and other tips. Good luck.

GLUE IT

I've done a several grill cloth replacements - tried hot glue on one - not a good result; staple gun on one - easy but I didn't like the staples. The last few I used brush on contact cement - it's messy but worked well. It holds well and I was able to get the alignment just right. Spray on 3M contact cement may work well but overspray would need be controlled so you only get the edge of the cloth, and the edge of the baffle board - you could do this with a masking jig - maybe made from cardboard.