hatepain- The Boston have internal adjustments for both tweeter level and for Xover frequency. Plus you could put a L-Pad from Radio Shack or even use fixed resistors if they were still too loud. I can help you lower the level.
NuBu- The impedance would only drop when both speakers were asked to play at the same time. For example, since the crossover on the tweeter on average takes away all the information below about 3500 Htz, the only time the tweeter actually plays is above that frequency. Since most of the amps load is seen by the mid/woofer it's really not a big deal for the amp.