i read somewhere that if you're using the passive crossover of your components .the Hu and amp crossover should be off or set to flat .is this true?

 

Need more details. Most HU and amp crossovers are sub/mid level only and for a component set you may want to dial in a high-pass of 80-100hz to keep your mids happy.

 

It can be beneficial to keep them on if a bandpass scenario is needed. (adding in a HPF to a component set for example)
However it is redundant to use multiple crossovers with the same purpose. It's not going to cross it better, but just create phasing issues.