Well different makers say different things.
For example on the Triamp MkII there is a 50W mode and in the manual there is no mention of impedance change. I contacted H&K and they said they kept it that way as the mismatch that results from running @50W, where you should usually connect the 16 Ohm cab to the 8 Ohm tap, if you stay with the impedance mentioned on the output, it's a safe mismatch that creates a bit of a more vintage/softer feel that they intend for the 50W mode.
Then in Mesa/Boogie manuals they usually say it's Ok to connect 16 ohm cabs to the 8 Ohm out, but the sound will be a bit less loud and punchy, and a bit darker. Which could be a good thing on a very stiff feeling amp.
Best thing is to have the option I guess. You can always rewire a 16 Ohm cab to 4 Ohm.
But wait - the internal speaker is 16 Ohm at the moment? Why don't you connect the external cab with the internal speaker and you will have perfect 8 Ohm for your output. However the internal speaker will be louder in relation to the external cab I guess.
