Best bet would be to pick up a second power amplifier, in the $200 to $300 range, for the outdoor speakers. You would not have any issue of low impedance with two speakers connected in parallel, and the outside system would be decoupled from the inside system so that no degradation would happen.
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How about just listening closely to the main system with and without the box in place, and decide for yourself whether there is degradation in sound? Let us know what you find.
I am in the process of solving a similar problem in a different way. I do have a separate preamp and amp (not yet installed) for "the rest of the house". But I don't want to buy (or house) additional sources. So my plan is to do "Y"s from all my sources into both preamps (one for my main system and one for everything else).
The advantage over your little box is that I will be able to play different music on the main and "rest of house" systems at the same time. In fact, the "rest of house" preamp (Adcom GTP-506) contains 3 zones and 2 tuners, so I could theoretically have several different types of music going at once!
If you don't need this multizone capability, the little box seems cheaper and better to me. If you can avoid purchase of a separate system, more money for upgrades on the main one (or for music)! Assuming you don't hear a sound degradation...
BTW, I believe the box is a "passive resistance network". The problem it fixes is that two speakers on one output look like one speaker with half the impedance. This can quickly be a problem for the amp. Others with more electrical background can say more. Here is a link:
Thanks for the responses so far, and for the link. I checked out the link, and it did indicate the selector box compensated for impedance and capacitance when running more than one pair of speakers. It also indicated that the "direct" output would have minimal effect on the speakers connected to those terminals. Other than the cheap binding posts, the unit seems quite well made. The listening test is a good idea. However, in addition to the effect of the box, it also introduces the effect of an extra run of speaker cable - from the amp to the box, in addition to the cable already in place to the speakers. I am using Alon's Black Orpheus as my main wires, and had to hook up some AQ Type 4 I had laying around from the amp to the box - causing a noticeable difference in tonal quality and resolution (degraded). I just purchased some used short Coincident wires to upgrade the run from the amp to the box, and am hoping they will help. My other option is to either cut my 25 feet long Black Orpheus (I don't need that length) or try to buy some new BO wire, but I got the Coincident for a reasonable price and thought I would try that first. I was hoping someone else had some specific experience with this selector box, but I have never seen one for sale here, or read a post on one, and the Rotel website was not clear whether it is even a current product - I got mine about 6-7 years ago. I will update this with some listening notes when I get the new wires in place.