What makes separates better than a all-in-one receiver, like the Yamaha RX-V661? Is it the built-in amplification that is sub-par?

If I decide to return/sell the LMC-1 and keep the LPA-1 amp, would a receiver with pre-amp outs to the LPA-1 do just as well as dedicated pre/pro? I know that for audio: good quality DACs are important, so if the receiver has good quality DACs and I bypass its amplification and use the LPA-1, is my audio quality going to be worse than a pre/pro's?

Ah, I don't even know what I am saying or what/how to ask...

Regards,

Rodney

Denon AVR-3312ci
Mains: M22 v2
Center: VP150 v2
Surrounds: QS8s v2
Sub: HSU VTF-2 MK3