Sensitivity Issues

I'm on the hunt for a speaker upgrade. My current amp is, well, modest (30 wpc Pass Labs). I've been advised to confine myself to sensitive speakers (more sensitivity in the house has cheered my wife). What range am I looking for, and how big a difference is there in a couple of db's, say 89 and 91? Also, if I drop down in resistance (8 to 6, or even 4 ohms), does this compensate, or do sensitivity figures already take resistance into account?