If I correctly understand that you want to parallel both the Niles box and the B&W's across the amps' outputs; theoretically this should be safe to do with a 1.5 ohm minimum amp load spec. It's a sonic compromise of the amp's damping characteristics, but this isn't a major issue unless performance is the absolute goal, which doesn't seem to be the case in this situation. Be aware however that 8 ohms nominal load can at certain frequencies drop to half or less of that value. Because the impedance curves of the two major loads are dissimilar you should still be within the safe operating area, but DO exercise restraint on the throttle unless the Niles is switched out-of-circuit (no speakers turned on).
Do I need to use a speaker selector?
I have two audio systems. My question is directed to my secondary system, which consists of a pair of B&W 805 speakers (8 ohm, nominal) and a "whole-house" speaker system comprised of 7 pairs of speakers, each connected to a Niles Impedence Magnifying Volume control. The whole-house speaker system is supposed to present an 8 ohm load to the amplifier. I am purchasing a PS Audio GCA 250 to drive the B&Ws and the whole-house system. Here's the question: do I need to use a speaker selector box, or can I hook the B&Ws and the whole-house speaker wires directly to the amp? If I connect direct to the amp, the load it will see may go as low as 4 ohm, nominal. PS Audio claims that the GCA 250 can handle a load with impedence less than 1.5. Am I missing something, or can I skip the speaker selector box and wire directly to the amp? Any thoughts or suggestions would be greatly appreciated.