My samson 88a sub is not working, monitors work fine. what could the problem be and how do I repair if it's a easy fix. thanks.

You can safely connect your powered monitor speaker to either the monitor output jacks or the tape output RCA jacks on your XP510. In addition, you may download the owner's manual here:
https://www.manualslib.com/download/515708/Samson-Expedition-Xp510.html

A snapping, crackling or popping sound?

One of the big capacitors on the power board is going South.

Fixing/replacing it is cheap and easy, just Google for the instructions specific to this speaker.

There is a wealth of information for repairing this speaker online, just poke around the interwebs.

Maybe train your voice to be louder https://www.youtube.com/watch?v=Oq4Iwr2Mzow

Call Samson to get a replacement or repair.

I have repaired speaker surrounds using repair kits from an internet company- 'Simply Speakers .com" The repair worked 100%.
I do not know if they have a repair kit for those 24" subs, but ask. ( wow -24" subs!).
If this info helps- please rate. Thanks...

Maybe a capacitor in the amp section is making this problem.The only way to find out is to check receiver in service and find which capacitor is bad...but this is not so easy

It seamns like its pretty commom =(
We should organize and complaint with samson!!
Read this:

http://www.gearslutz.com/board/low-end-theory/314545-samson-rubicon-r5a-problem.html#post5794093

Its prob a dry solder joint problem.

http://www.gearslutz.com/board/low-end-theory/314545-samson-rubicon-r5a-problem.html

If sub has two signal outputs and if the monitors are full range (passive crosover built in), put mixer outputs to the sub and signal outputs of the sub to amps in.If not mixer outputs to th amp and its signal outputs to the sub in.Or you can use main out of the mixer for the monitors and rec out for the sub.Its just few XLR or jack cables extra.Just make sure that the mixer,sub and the amp are on the same power phase otherwise you'll have brrrrrrrrrrr on the speakers