First thing I'd do is make sure that the polarity is correct on the drivers. Best way to do that is to apply a DC voltage to the drivers and see that they move in the same direction. If not, reverse the connections on one driver.
However, if you want to replace the drivers, my first choice would be JBL D140F's. Being vintage drivers, they can be rather pricey. There are lots of other low Q bass drivers available that would do as well, or nearly as well. I'm a big JBL fan, so take that into account, but the 2032 works very well in a sealed cabinet, and they're usually available cheap. K140's are nearly the same as the D140's. E140's are similar. If you want more output, and are willing to settle for less bottom, things open up even more. D130's, K130's, E130's, 2225's, 2226's and the very high output 2220 driver are all possible choices. No doubt I'm missing some excellent alternatives.
Other excellent drivers come from Altec and ElectroVoice, but I'm not familiar enough with them to recommend any specific models. Eminence makes more drivers than any other company in the world, and surely has many great choices. I've heard good things about Beyma, Fane and B&C.
Sunn used JBL drivers for a long time, but had stopped by the time the Concert Bass and Lead amps came out. Fender used JBL's as well (that's what the F in D130F and D140F stands for), and it's possible that the bassman originally came with them. I think they also used Jensen drivers, but I could be mistaken. I'm not a Fender historian by any stretch.