I want to buy an av receiver for my dvd player. I already have the speaker system with 5 speakers - Front, Center, Surround (each has 4 ohms) and 1 active subwoofer. I need some informations before I buy. What av receiver should I buy for 4 ohms speaker system? can Active subwoofer connect with the av receiver? What is the pre-out of the av receiver for? my budget is limit to not more than 500 US $ for the av receiver.