I can tell you that neither inverting polarity nor using a speaker with an impedance higher than the amp's rating will do any harm, unless you drove the amp into clipping, but the speaker's tweeter is usually the first to go.

Perhaps you accidentally created a short with a wire strand.

"Perhaps you accidentally created a short with a wire strand."

How would I check this? I thought I "confirmed" that the center channel was blown by switching one of the other speakers to that input (nice connector for each speaker). The center channel speaker itself will work on another channel.

I meant that you might have shorted the center-channel speaker terminals with one of the speaker wires. When using stranded wire, it's important to make sure the strands are twisted neatly, and to check that no stray strands are bridging terminals after connecting them.

When you read product specifications, you are not reading reviews. When you read reviews, you have to read enough by a particular person to get a sense of what they like and don't like, or if they are responding to advertising dollars....

The specifications follow federally mandated rules so they really are specific specifications!

And yes, an amp could be optimized for 8 ohms and save some money. It still would put out power at 4 ohms, maybe as much or less, maybe at slightly higher distortion.

By the way, connecting a four ohm speaker to an amp optimized for 8 ohms would not blow the amp or the speaker until the volume was turned up to the point where the amp was in trouble. It could clip the tops off of sine waves, which introduces huge amounts of power at high frequencies (clipped sines look a bit like square waves, which ave lots of high frequency power); that is why tweeters blow first when an amp is run too hot. Too much current for the amp could run through the four ohm load, which should shut off the amp when it gets too hot, but could also blow it. But at reasonable levels, all would be well.

Those old boat anchors I used to call them such as the early Mrantz's and Sansui recievers no doubt had a lot less failures as a result of connecting 4 ohm speakers. I still have a 2270, and the thing is a tank. It also weighs about as much. But pop the hood and you will see a massive transformer, and some very beefy caps. It is also completely solid state. It was built a lot like many of the pro power amps.
Interestingly enough however, many of the newer models from Crown have employed innovative technology that ends up delivering high power at low impedances in a lightweight package. Surface mount technology plays a part, but I think there is more going on. Cooling and heat sinking is almost as important as performance, and unlike a home application, in the pro environment, the fan noise isn't a factor. The challenge in building home products is doing it without a fan involved