The need of an amplifier for 6.5” speakers depends on your requirement. The standard manufacturer recommended speakers would be just fine if loud music isn’t particularly interesting to you or your requirement. However, these speakers can spew just 10-15 W of peak power. So if extremely loud music is what you require, an amp will be recommended as it has a high RMS (Root-mean square) rating which means the speaker can emit a lot more sound from the voice coil.