Looking at the manual: http://www.americanmusical.com/ItemF...lve_manual.pdf
The back of the amplifier has two outputs. One, I assume, is now used for powering the combo speaker. The other is free to connect to an external speaker. Since all information asserts that you can run the amp at 200 W maximum to the combo speaker, we must assume it is rated at 8 ohms.
If you like how your combo sounds, why not get an Ashdown extension cabinet of the exact same speaker configuration you now have, also rated at 8 ohms? Powering that one off the now vacant speaker output jack, you have a total (parallel) load of 4 ohms. This way you will be able to utilize the full output of the amplifier.
I hope this was the required Aspirin for your brain.