My rear speakers crackle a lot but not always. Sometimes the system sounds great other times it it's not impressive. I have infinity speakers, head unit, sub and amp. Some guy told me I need to increase the rear cross over frequency. Is this most likely true? Can do that right on the amp? Are the speakers blown?
Thanks,

 

you could always just through a 1K ohm resistor onto the negative side of the speaker jack. That would cut down the power going to your speaker, hence not over powering the speaker.

doubt they are blown, if you have replaced them already. You can push down and up on the cone, there should be no crackling sounds or places were you can FEEL it grab or catch on something.