Impedence and Speakers

One of my boys asked me about adding a speaker onto his current bass rig, a Fender Bass Man 250 with 1 15" speaker in it. It has 2 parralel 250W outputs at 4 ohms, with one input used for the combo speaker. He wants some shake, not some thud and wanted recommendations on what type of external speaker and wanted me to explain that ohm shit to him. Then I realized, I needed some one to explain that ohm shit to me agian.

A good impedance calculator for speakers and not math wizards- 2 speakers at 4 ohms in parrellel is a 2 ohm load. the same in series would be an 8 ohm load.

I started doing the math and adding either 8 or 4 ohm speakers doesn't work out with the specs on the head. I found a forum here that states that in order to use this combo amp to power external speakers, you would need to unplug in the internal one.

The Bassman 250 Manual pdf-

Conclusion- If you have a combo amp, you most likely can not add another speaker cab on cause the combo is set up so the one speaker is the load and isn't expecting other cabs to be used.