some subwoofer questions...

i am a little confused about subwoofers and crossing over and such. i have a hsu vt-3 sub connected to a denon avr3802 via the sub pre-out. i have the reciever set to large for my mains (paradigm st60), but from reading other posts should probably have it set to small. if using the denon's crossover, then my only option is crossing over at 80Hz. if i switch my mains to small, then do i also set the subs crossover at 80Hz? if i set the subs crossover at 60Hz, than do i not get anything between 80 and 60Hz? i know room placement is critical, but what suggestions do ya'll have for my set up? also, does the volume on the denon control the volume on the sub? so when i turn the volume up, the sub should get louder as well?