Ok, I am assuming that this is a sealed cabinet (there are some Bassmans that are also ported, as well as reproductions and modified cabinets).
For this to have any meaningful low end, you will need a bass driver that's optimized for a sealed cabinet and not a ported cabinet. Most bass drivers are designed around ported cabinets (performance suffers in a sealed cabinet, that's why the complete specs are important) but there are a couple of options that work ok in a sealed cabinet:
1. The Eminence BassLite 2012 is a good middle of the road option with a nice balance between LF extension, mid voicing and sensitivity
2. The Eminence BP-122 is another option, a little better LF extension but darker in the mids and significantly lower sensitivity. For using with the Bassman 135, it wouldn't be my first choice. If you have more power and lowish volume requirements, it would be ok.
3. The Eminence Beta 12A is also ok in a sealed cabinet, kind of in between #1 and #2, not the brightness of #1 but brighter than #2.