The proliferation of small "bookshelf" speakers wired to the 8-ohm tap of an amplifier requires careful attention to the load being generated on the line.  While most reputable manufacturers design their amplifiers to handle plus or minus 50-percent variations in load factors from their rated capacities, even this wide range is easily exceeded.

Amplifier abuse leads to failure although it may not happen immediately.  This situation is similar to the story about the fellow who claimed to be able to defy gravity.  As proof, he jumped off a 20 story building and was heard to say "so far, so good" as he passed by the 10th floor going down.

At least, when working with an 8-ohm line, consider using the series wiring method shown in the schematic below.  Then you can simply add the impedance of each speaker to derive a total load for the line.