1. I do not have answers for this.
2. Low pass means that frequencies below the cutoff point will be passed through. This is usefull for woofer. High pass means any frequency above the cutoff will be passed through. This normally applies for tweeters.
3. The idea is to set your gain to a level that will not cause your amp and speakers into distortion. Set it at a minimum before raising it up slowly until you can hear distortion. But, this is not that easy especially for woofers.
4. This I assume is to use the amp built in crossover. If you have an electronic crossover, there might be no need for you to use the amp's internal crossover. If you don't have one, this can be used as mentioned in answer 2. However, please be carefull as you don't want to send high frequencies to your woofer and low frequency to your tweeter.
Just read the manual (if it comes with one) to better understand the operation of your amplifier.
Hope this helps.
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