Paul,
According to the Audio's 42nd Annual Equipment Directory, the Gershman SW-1s have a sensitivity rating of 87 dB/watt, and I didn't see any specs on the crossover, although judging from the low frequency range of the intended Gershman satellites (to 40 Hz) the crossover may be set low enough for you to use as-is and let your main speakers roll off naturally. I would contact Gershman for information on the crossover point. The difference in sensitivity, however, may force you to use the EVS attenuators to turn down the main speakers (which appear to be more efficient), rather than the subs -- so you may in effect be adding a passive preamp in line with your main speakers, to make this mating work. Also, depending on the crossover point of the sub's internal crossover (if any) you may need to remove the crossover (if it's set too low) and get an adjustable external crossover to obtain optimum results. It sounds like you might be in for a little experimenting. Good luck.