Hi Nick,
I think it would happen only when more than one pair of speakers on the switcher were engaged. It wouldn't be a big power loss. You could order one and try it out temporarily with a couple pairs of speakers to see if it works OK and doesn't cause your Onkyo to shut down.
My comment on the fidelity issue is based on the idea that the resistors may interact with the impedance of the speaker in an unpredictable way. This is speculation on my part based on comments from Axiom's chief engineer, Tom Cumberland, when I asked him about multiple speaker hook-ups and constant-impedance switching devices.
Alan
