Hi ! I want to buy the Onkyo HT-S9400THX and add a subwoofer unit. Questions: 1- can I add one subwoofer without make any adaptations of something like that? 2- I was thinking in 2 subwoofers: Polk Audio PSW10 and Onkyo SKW-204. They are good? Can I just buy one and connect it in the S9400THX receiver? Thank You!