A speaker selector is your only option, but IMO five pairs of speakers is probably more than you should put on an amplifier. Sure, the speaker selector will maintain the proper load for the amp – probably 8-ohms – but the amp’s power will be divided out between the speakers. Considering the 1913’s 90-watt rating, this means that each speaker will be getting no more than 18 watts at most.
So while the load might be correct, you might well be running the amp wide open, or close to it, to get the desired volume levels in the secondary zones. Naturally this will only shorten the amp’s life span. (Naturally, how efficient the speakers are – or not – will be a contributing factor to how much power output is demanded from the amp, as will how often you use the second zone.) Adding insult to injury, the speakers’ volume controls will suck out additional power. The speaker switcher itself may also be a power soak as well, as it uses a resistor network to maintain the 8-ohm load. So it’s highly probable that that 18 watts-per-speaker figure is optimistic .
Really, the best way to power so many speakers is to use a multi-channel amp. Unfortunately the Denon has no line-level outputs to accommodate a second-zone amplifier, which is surprising for a receiver in its price range.
Wayne A. Pflughaupt