Get yourself a speaker selector with a built in protection circuit.
Here's a copy of a post I made on this subject in another thread:
If you wish to drive more speakers than your amp/receiver was intended to drive, do yourself (and your amp/receiver) a favor and get a speaker selector with a built-in protection circuit.
As others have pointed out, the more speakers you drive in parallel, the lower the impedance your amp/receiver sees, and the lower the impedance, the higher the current draw on the amp/receiver.
A properly designed speaker selector will have a protection circuit with some large wire wound resistors that bump the impedance back up in the range that will be safe for your amp/receiver. Adcom, one of the manufacturers of these devices refers to this as Minimum Impedance Protection, which is exactly what it is.
Speaker selectors, with protection circuits are available new from several manufactures. Niles is the most well known. They have been making these devices for decades. New prices for Niles speaker selectors start at about $100. There are cheaper brands (Monoprice, Pyle, etc.) that start at about $30 - $35, but if you want to save some money, watch for them to show up at your local Goodwill store.
A ton of these were sold back in the 1990s when the "whole house" sound system became a big thing. I see Niles speaker selectors at my local Goodwill all the time for $4 - $6. I just picked up an older model Niles SPS-1 this past weekend for $3.99 at my local Goodwill store. I like the look (walnut cheeks) and build quality of this older model:
The newer, low profile Niles SS-4 is the one I most often see at my localGoodwill stores:
There are more elaborate models that include individual volume controls for each pair of speakers (so you can match the volume of speakers with different efficiencies), but I rarely see these at Goodwill.
At the very least, if you are going to run more than two pairs of speakers in parallel, get one of the basic models with the built-in protection circuit.
