Can I buy any audio reciever for my home theater speakers?

I currently own a Sony DAV-DX155 home theater system and am looking into buying another receiver for this unit. Can I keep the speakers i have (that are wired into the walls) and just buy any new receiver (or maybe just a sony)? I need another receiver so I can enjoy full digital 5.1 surround sound with the devices I own.
One extra thing, how do i hook up the sub woofer that i have on the unit? All the receivers I looked at had no + and – output for the sub woofer.

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