So I have been reading here and there...but a question remains. If someone has a 7.1 AV Receiver, ie. a Harman kardon...then why the need for a separate Amp. I guess my question is really....what are the benefits of having a separate amp (nad 7.1) plus a preamp? Just the amp power or is there something I am missing. My hk will do 120w per channel and that seems adequate...so why get a amp?

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Denon 4308
M80x2,VP150,EP600,4xQS8