Here’s the deal...in those days, early/mid 60’s, most amp companies, save for Ampeg and their B15, didn’t really put that much research and effort into coming up with proper bass amplification and cabinets. That started to happen just a few years later courtesy of Sunn, Acoustic, Ampeg(who really rose to the occasion), and to lesser extent, Kustom. Vox, another big name of the era, didn’t really get it together. Fender was surprisingly uninterested or misguided with their bass amp design. Although, yes, in the 50’s they introduced the first mass marketed bass guitar, and offered the tweed “Bassman” amp to go with it, it was inadequate as it was an open back combo(for some reason they didn’t figure out that bass required a sealed cab). The earliest models actually featured a single 15”, but they quickly switched to 4x10’s to prevent blown speakers, but ironically it was a bigger hit with guitarists. By the time they switched to the separate head/cab ‘piggyback’ format, the all-purpose Showman model, paired with one or two 15”’speakers, was the top of the line with higher wattage. While the then-new Bassman head did have a tone section that was more bass frequency friendly, it was only 50 watts, and came with a smaller 2x12” cab, Fender assuming that it would be adequate for bass. The similarly configured Bandmaster guitar rig was its matching sibling. The major problem was that ALL Fender cabs at that time were no more than 12” deep, owing, I suspect, to materials and manufacturing efficiency. The Showman boxes were taller and wider, but as far as bass was concerned, the Bassman, with only the dual 12’s and a shallow enclosure, just couldn’t deliver, especially after the Beatles hit, crowds got larger, and there was a demand for louder amplification. Fender did eventually increase the size of the cab, but stayed with only two 12’s for awhile, too little too late. That’s when all those other companies stepped in with better ideas. All that being said, your cab can certainly make for a great guitar box with the right drivers, or, with matching bass specific 12’s, a low volume bass cab. Just don’t expect too much low end from it in that capacity. If that’s what you want to do, I’d suggest selling the JBL(which, yes, is fairly heavy)and taking the money and getting some Neo’s to cut down on weight. But then, you might have a use for the D131 in a single 12 guitar combo. It’s up to you. What kind of head do you have for guitar purposes?