In any event, modern amps have such high damping factors that even such an effect is meaningless unless you are running the amp way below its rate impedance... This is audiophyle nonsense, no offense intended...

Series, parallel, whatever... Series wiring will give you no increase in sensitivity, but will double your power handling. However, as your friend mentioned, there will be crossover interactions when wiring mult-way speakers in series with each other. Single drivers are no problem.

Adam