All I can say is that you're probably right that the Solos has 8 ohm speakers. Sunn was usually good about impedance matching, and switching the transformer tap used. My guess - and I stress that it is only a guess - is that the internal speakers combine to 8 ohms, however it's done, and that plugging a second cabinet into the external speaker jack switches the output from the 8 ohm tap to the 4 ohm tap and puts both speakers in parallel for a total 4 ohm load.
As for the DC stuff, it goes like this. Speaker impedance varies quite a bit with frequency, so the nominal impedances are really only approximations, at best. However, if you measure the DC resistance of a speaker, you'll get a value close to, but lower than the nominal impedance. So, if you measure the DC resistance and get something like 6.3 ohms, it's reasonably safe to assume that it's an 8 ohm speaker.
