That's a shame and a real waste of money...but a good lesson to pass along.
The gain stage on an amp very often misunderstood. It's not a volume knob to make an amp put out less power. Turning input gain down will not protect speakers that are rated for much lower power. When you turn your head unit up, regardless of your gain settings, your amp will still have the potential to produce the rated wattage and blow speakers that aren't rated for that power.
Gain is a setting for the front end input of your amp. It simply gives the user control...to make sure you're not distorting signal on the input side.
Your speakers should always match the output rating of your amp for more than one reason. On the other side of the spectrum...under powered speakers will never sound as good as they should.
2013 Cross Country (Houston/Katy Texas)
People think I am too patronizing. (That means I treat them as if they're stupid.)