HELP WITH SETTING UP DAC TO PREAMPLIFIER

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Hey guys I’m asking for my father he is using a pair of infinity reference 60 speakers with a Denon Poa 2800 power amplifier and also using a Denon DAP 5500. My question is I’m assuming the Denon DAP 5500 all ready has an inbuilt DAC in it. The sampling frequency for audio and cds etc is from 32khz, 44khz, and 48khz. If I was to get a DAC seprately would I still be able to setup it up to use together with my Denon DAP 5500 digital preamplifier? Would it bypass those sample rates and give me a higher sampling rate if I had a separate DAC? And would sound and performance be increased? Or would the inbuilt DAC of the Denon DAP 5500 prevent me from using a separate DAC as it would just automatically activate its own DAC when operating ?

Cheers

Tony