Semi,
your questions have already been answered within these forums several times over. You may want to try doing some searches for sub setup and QSx positioning or other related words.
In general:
You need a Y connector for your RCA cable from your receiver to the 2 RCA plugins on the subwoofer. This is pretty cheap at any electronics store.
Two ways to setup your sub cross.
First method: Set all your speakers to "small" and turn up the sub cross all the way it can go.
Second method: Setup all your speakers to small except your main speakers, leave them as "large". Then turn your sub cross to 80Hz and listen to music. Turn down the subwoofer cross until you hear a good integration of sound between the sub and your main speakers. Considering you have the M80s, this crossover level might be quite low. I have the M60s and an EP350 with my cross set around 50Hz.
Test both methods and use what you prefer as the best sound.
Lastly, to clarify, the 'small' and 'large' settings of the speakers as set in your receiver does not determine power supplied. It determines whether the receiver's internal crossover circuitry will be used or whether the speaker will be allowed to utilize its entire range of sound below 80Hz. If you set the speakers to small, the receiver cross is usually around 80Hz and is only used if you have a subwoofer which typically takes up the duties below 80Hz.
However as i mentioned, try both methods above to find out which sound you like best and go with that. Play with it! Have fun!
