This is a reproduction of an article by eric_B on TalkBass.com on how he modified his Fender Rumble 100 combo (100 watts, 1x12″ speaker) so the combo could be used as
For a while I thought it would be cool if I could use my Fender Rumble V3 100 as a cabinet for an external amp. Following an idea of @AstroSonic, I decided to do this mod and also be able to use the amp to power other speakers. While I was at it, I also upgraded the stock speaker to an Eminence Deltalite II 2512.
The essence is that you disconnect the internal cable from the amp to the speaker and create a loop on the outside of the combo with 2 connectors so you are free to patch.
Here is a set of pictures of the process.
The internal amp is going out the right connector and is looped back into the cabinet through the left connector which is wired to the speaker.
Final word: it is a pretty straight forward mod, it works fine and looks not too crappy (at least IMO). I just need to put 2 stickers on it to mark the amp and speaker connectors. And be sure not to connect an external amp to the internal one, that probably won't be a good idea!
One thing that I found a bit annoying was that the Neutrik speakon/jack combo connectors are not pure black but have a greenish tint. Though it looks worse on the pictures with the flash light than in real life.
I'll see how I like it on the long run and maybe I'll mod my V3 500 combo as well.