i've seen alot of mentions of pre-amps and i really don't know what that is. what does a pre-amp do? i thought i have seen posts, here that people use old recievers as a pre-amp. i currently use a denon 3300 as my main. i have an older DD sony receiver that i only use when i want to watch DD laser discs. so can i use this a pre. and how would i go about doing it? thanks.