Trying to wrap my head around cross over, avoiding black holes??

[aesnakes]

Hopefully someone like Big Daddy will see this because i just cant wrap my head around it at all.

I really care about my sound, even though my system isn't amazing I want it to sound its best.

I was recently playing some DTS audio CD's and some other videos and I noticed on somethings my sub was almost silent...yet on blu-rays it was at a good level. As soon as I switched my rear speakers to small on my amp I started hearing a better response in the sub.

So I have the Polk 6750 setup, and I added 2 Polk M20's and my receiver is a Sony STR DG820. The sub Set up at half volume and the freq is set at around 130hz and can go to 160. My front speakers are set to large, the center is at small, and cross over set to 170hz, the 4 rears are set to small and cross over at 140hz.

A lot of these are defaults, and you can not set the crossover unless the speakers are set to small......so should my monitors be set at small and the crossover changed? Should the other speakers be changed? IM just lost please go easy on me hopefully I can learn if I am doing something wrong.

Thanks
Rob