Looking for earphones under $50 to be used for mostly rock music. Have no knowledge on what makes an earphone "good" so im coming to you guys for help and would love it if you could explain so I could learn as well c:

Heavy metal fan here
I don't know about SoundMagic E10, but Piston 3 aren't good for rock. The bass is too boomy and consumes all the details. I had XBA-C10 before, they are much better and cost about the same. The sound is clear, open and neutral almost all the way, but drums tend to sound a bit too tinny and there's a severe lack of "oomph" spectrum of bass. They will fail miserably in any music that relies on it.
 
Even for the rock, you will have to use EQ to make C10s sound right. But it is possible. Pistons are just hopeless in that regard, there's no way to make rock sound tolerable in them, especially compared to XBA-H1 that I have now.
 
My first recommendation, hope it helps!