Well John, the reason that the amp driving the mids/highs didn't shut down is likely the result of two factors: the impedance of the M80s in those frequencies varies from about 4-15 ohms, with an average of about 8-9, so that made things easier; in addition, it's likely that the highest power demand on that particular material occurred in the bass range, so that amp had to handle very little of it.
As to the bass section amp, no, there isn't any difference in the impedance it faced(and current required)before and after; from the published graph it's very close to the nominal 4 ohms, and nothing's been changed, since the bass drivers are of course the same and the crossover circuitry hasn't been changed either.
As to the observed higher setting before shutdown in the bass, there shouldn't be any significant difference if the conditions were identical, since no additional power capacity has been added. The temperature and/or current limiting circuit should shut it off at about the same setting. Possibly there had been longer operation and the temperature was higher when it had shut down at +4 on the previous occasion.
Enjoy the music, not the equipment.