hi, I suspect the issue with plugging them in right now is not really knowing whether, after 40-50 years, they will work properly or possibly cause damage. With a speaker system as potentially valuable as this one, it might really pay to learn a lot about the system, take inventory stock of the drivers and their condition (open the back panels, take pics and record model and serial numbers, dismount the woofers and check cones for condition and movement), before trying to power up the system.
Also, since your system may have the JBL energizer/equalizer in lieu or maybe in addition to stock crossovers (look for a 2-4 metal boxes with 6 to 8 wires routed to it/or its terminals and a round metal JBL knob), the wiring may be a little more complicated than the typical 3 way system.
Best to check the drivers using a known lower power amp just for checking purposes till the energizers can be checked out, maybe separately. I know, if the amp is there, there probably isn't another easy way to run the system, especially if there is no other crossover to properly route appropriate signals to the drivers.
BTW, the JBL literature says systems would be configured by 150-4C or LE15 Woofers, 375 mid drivers/horns, 075 bullet ring radiators and 4 dividing networks. Of course, with the energizer in there, there should be some differences. Come to think of it, these energizers would be usually two channel. IF they're in there, how many energizers are actually installed, 2, 3, or 4?
http://www.lansingheritage.org/images/jbl/catalogs/1967-home/page04.jpg
Regards,
Bart