One thing for sure, it is not all about the box build -- the JBL LX300 speakers are performing back-up duties, while the Canton GLS is on a working vacation. The LX300 were drafted off Craiglist.
The LX300 are from around 1993 and listed for $395 a pair, they have a 6 inch cast frame woofer and a 1 inch pure Titanium tweeter. They are 16" h x 9" w x 9" d and the front baffle is 7/8" with the rest of the cabinet 3/4" and they weigh 18 lbs each. They have a second order crossover and even a poly cap. Now for a cabinet like this, JBL still used spring clip connectors.
The JBL LX300 are over built for the sound quality that you get. And compared to the Canton, they lack in overall resolution. The JBL has detail, but is lacking in definition and depth. They are bright compared to the Canton, and the Canton even has more bass. The Canton 6 in driver diameter is smaller than the JBL. The JBL is limited in off-axis response, where the Canton is much better. JBL seems to have concentrated more on central imaging. The Canton tweeter is more smooth and refined compared to the JBL titanium tweeter.
They Canton may look and feel light-weight, but their sound is heavy-weight compared to the LX300 -- JBL LX300 sounds like a speaker with potential, but seems like they were rushed to the market. When the Canton comes back, the JBL will be put on waivers.