If the internal speaker is 16 Ohms and the speaker in the extension cabinet is 16 Ohms, the with the paralleled speaker output on the Classic feeding them both you will get 8 Ohms. Yer good to go.
Let's review: If you wire two speakers in parallel you will divide the impedance, which results in an impedance of 8 Ohms if using two 16 Ohm speakers. If wired in series the impedance will double so two 16s would equal 32 Ohms.
Yeah, I'm curious about the 8/16 ohm thing. I do think that higher ohm speakers are more efficient and sound a bit better, so I can see why you may want to do it, but I'm confused by why you think it matches.
Poppycock. The rated efficiency of a speaker (listed in dBs) is different than the rated impedance (expressed in Ohms).
I also challenge you to hear the difference between a Celelstion Greenback 8 Ohm model and the 16 Ohm version.
When presented with a different impedance load an amp may perform more or less efficiently. Not the driver.