If you'll note in the 7100 specs, as the impedence of the load drops from eight ohms toward four, the power delivered increases. (See Ohm's law and variants elsewhere.) For various real world amplifier reasons you would find that an impedence vs. power graph would not be strictly linear, but the principle still applies. As a result your less efficient speakers will be recieving more power, all settings being equal. If you were in a perfectly ideal environment with all speakers equidistant from your position, I imagine that a +/- one db tweak here or there would bring all levels to the same point. You are likely to find that the variables of your listening environment will have as great or greater impact on balancing levels at your listening position(s) than the impedence/sensitivity issue.