There have been some questions about this amp. One question on this forum was if the BXR100 was the same amp as the BXR200. Halving the impedance of the speaker load in the 100 would cause the amp to develop 200 watts. I just looked up the schematics for the 100 and 200 and they are not the same amp.
BUT I did learn something in my search. The manual for the BXR100 states the speaker is 8 ohms. The schematic shows a 4 ohm load. A search on the internet keeps referring to 4 ohms. Replacement speakers sold are 4 ohms.
If you wish to run the 8 ohm 4X10, you will need to do this. You need to open your amp and see if the 15 is 8 or 4 ohms. Disconnect the speaker and measure across the terminals on the speaker. The resistance will be around 6 ohms if it is an 8 ohm speaker, and around 3 ohms if it is a 4 ohm speaker.
If it is a 4 ohm speaker, you could replace the speaker with an 8 ohm 15, and add an ext speaker jack in parallel, allowing you to use an 8 ohm extension speaker.
If your amp IS designed to run into a 4 ohm load, changing to an 8 ohm internal speaker would allow you to still use the amp alone if you wanted, but into 8 ohms the amp would simply develop less power, and run a little cooler. Then for the full 100 watts, use an 8 ohm extension speaker.
If the internal speaker IS 8 ohms (and perhaps there is an earlier or later design than the schematic I got from Fender) then to run an 8 ohm extension speaker, you would have to add a switching speaker jack that would disconnect the internal speaker when using an 8 ohm extension speaker.
Feel free to e-mail me to discuss this further, if you don't want to keep talking about it here. I'll keep an eye on this thread.