The L300 was a good speaker and is quite collectable, so you might make sure your changes are reversable. But I think that JBL's crossover designs of the era are a weak point, and there are significant improvements to be had with revisions.
For a in-depth discussion of the specifics, I'd inquire with the guys at the vintage JBL forums:
http://audioheritage.org/vbulletin/
For a in-depth discussion of the specifics, I'd inquire with the guys at the vintage JBL forums:
http://audioheritage.org/vbulletin/