The original Dual Showman used the JBL D130 speakers, which was really a stereo hi-fi speaker not designed for musical instruments. Then, after Fender had them pretty consistently blow, they asked JBL for the same speakers that would live in musical instrument use. Fender re-deisgned the D130 specifically for use with Musical instruments and called it the D-130F. It had a smaller frequency range than the Hi-Fi stereo one did but was perfect for guitar and also pretty good for bass. Fender still had trouble with blown speakers in the D130F when used for bass and JBL went back to the drawing board and modified them with again a smaller and a lower frequency range then the D130F and called it the D140F. That speaker was excellent for Bass and lived just fine.
So it's possible that you may have any of 3 different 15" JBL speakers in there. You'd have to see the back of the speaker to tell if it's a D130 or D130F, which means you'd have to remove about 50-screws off the back of that sealed cabinet. I had a 1966 Fender Bassman that I bought in 1966 and it had Jensen 12" speakers in it, which I blew 3 times before getting another amp. So I know about removing all of those screws. Just think back to the 60's when they all had to be removed and installed with a manual Phillips-head screw driver. No automatic screwdrivers or screwdriver bits used with drills...ugh!
The good news is that if you shine a light through the front of the grill cloth, you should be able to tell if its the D140F speaker because the the D140F uses a ribbed cone. If it has ridges, it's the 15" JBL that was designed specifically for bass and will outlive the other two quite well. It came out in the mid-60's and if I recall correctly you could order those in the Dual Showman in 1966 when I bought my Bassman amp.
In 1971 while I was playing bass for a touring nightclub group, I blew my third Jensen 12" speaker out of my 1966 Bassman amp. I literally went to an electronics shop that had musical instrument speakers one day, bought another one, had my lead guitarist solder it in place, and played that night. But that was it for me. Both the keys and the guitarist had purchase Vox Super Beatle amps because Thomas Organ was closing them out. Those 120-watt amps just buried my poor 50-watt Bassman amp. Plus, if I tried to turn my Bassman above #7 on the dial, the speakers would start making farting sounds. So I had to get a new amp and I decided to get a Dual Showman.
We were playing in Lincoln Nebraska at the time and I went to Hospe Music to see if they had one. They were sold out but called their store in Omaha, NE to see if they had one in stock. Bad news was they didn't. So the salesman took me to their "Bass-ment downstairs which had all the Bass amps in it and they had a slew of them. I played my way through almost everything and just couldn't find something that would reach the level I needed without distorting. Then the guy showed me this comparatively small piggy-back Standel amp along the back wall. It was shorter than my Bassman but easily twice as deep. It had a powered 215 cabinet in a bass reflex design with 4 round ports mounted vertically next to the speakers.
Guy told me I could play a low E on it with the amp dimed and it wouldn't distort...so I did. It was crazy loud and shook the walls. Then he offered it to me for $950 ($200 off) because it was last years model. Then I looked at the grill-cloth on the head which had about a 2" snag in it. He immediately offered me another $100 off. Told him I'd take and asked him if I could trade in my '66 Blackface Bassman and his eyes got real big. Said he's have to see it so, I went to the club and got it. When I brought it in he looked like he's died and gone to heaven. He offered me $400 trade-in allowance on it (black face Fenders were rare at the time and highly desirable). Since I had paid $449 for it brand new in 1966 I jumped on that offer and after my trade-in, for $450 I walked out with an $1150 Standel amp.
It was a monster that projected sound like you wouldn't believe. So it was a 200-watt amp that would flap the bell-bottoms on my pants if I got within 3-feet of it with the volume set at 9 o'clock, and I no longer had a problem being buried by those Vox Super Beatles. I could easily bury them now if I needed to...but I'm not that rude and they were friends.
So I'm actually glad that they were out of the Dual Showman amps that day. They were great sounding amps but I was super happy with my Standel amp until someone stole all my touring gear in 1976 including my Standel amp and my 1965 Jazz Bass. I might still be using that amp if I still had it...if my back could handle it. It was not a light weight amp but I loved that amp.
I would recommend that you check to see which 15" speakers you actually have in it. A bunch of folks preferred the D130F but didn't drive them hard (medium-sized venues). Guitar players preferred the D130F and when you bought a Dual Showman you could specify either the D130F or the D140F. I shared a stage with a group at the Commercial Hotel in Elko, NV in 1972 and the bass player for the other group was using a Dual Showman with the D140F speakers in it. He had no trouble filling that large room and he sounded good through it. But you really need to find out which JBL's are in it because the prices I've seen for re-coning blown JBL D140F and D130F speakers are around $400.
Perhaps
@agedhorse will pop in here. He's very familiar with them and I believe he used to re-cone them too. If he tells you anything contrary to what I've said here...go with what he says.
