Cant understand Meridian Gear

Need some help understanding Meridian. First of all I'd like to purchase a 5.1 or similar processor preamp...don't care about video switching etc. I've looked at the Meridian gear, but can't figure out what the difference is in their units. Is it a fully decoding and functioning Pre processor or not? Some units are labeled as controllers, but they require an additional unit for DAC, don't know? Are some units stand alone? I have some monoblock amps, bluray player and speakers. Need to know which used Meridian to buy (single Unit only) Thanks