Need advice on digital inputs

The way you have it now you’re using the DAC in the Cambridge — the giveaway is you’re using the analog outs from the Cambridge so the digital to analog conversion has already been done.  If you use the digital out from the Cambridge to the NAD you’ll be using the NAD’s DAC.  If you want to want to try the NAD’s DAC to compare this is a decent digital cable that’s not too pricey (don’t use a crappy digital cable because they really do matter).  Hope this helps.

https://www.ebay.com/itm/264256579076

With the analog outs on the Cambridge connected to the analog ins on the NAD, you are using the Cambridge's DAC.

To use the NAD DAC, connect either the optical out or coax out on the Cambridge to the NAD input.  The coax connection should give you a better transfer with a decent digital coax cable.  (Do not use standard analog RCA cables to connect the coax connections.  A "digital coax" cable has a different design and different impedience values.)  If you don't have a digital coax cable and still want to see how the NAD DAC sounds, then you can use a optical cable.

You may need to go into the setup menu of Cambridge and tell it to use the optical or coax output.

@soix     you beat me to the answer....

If i use the digital out on the Cambridge ( coax) what exactly am I connecting it to on the NAD? The NAD doesn't have any digital inputs.

The NAD C379 has two optical in and two coax in. Not sure what you are not seeing. Read the manual.

It was stupid of NAD to label the RCA/SPDIF digital inputs as coaxial in as that’s a general term that could mean anything, and for exactly this reason pretty much the whole world calls it a digital input but yes that’s the input you’d use. 

@soix 

Correct me if I'm erring.

RCA is analog, with R and L connections. Coax is digital, using only one connection. The NAD has two coax for two different connections, not to be used together like RCA. 

@baylinor  More experienced people would know that, but obviously as with here it can needlessly confuse people who are newer to the hobby.  Just call it a digital input like virtually everyone else does because that’s what it is and makes it perfectly clear to all.  Stupid. 

Thanks fellas I appreciate all the help. 

Fellas if I could bother you with one more question...there is a big difference in volume depending on which input I'm playing the source material on, when using the dac in the Cambridge with the volume set at 50% it is considerably louder than using the dac in the NAD with the volume still at 50%. Any thoughts on this? Again thank you for the help. 

There is an extra gain stage when using the analog output of the Cambridge to analog input of the NAD. Adjust to the same playback level, compare, and use whichever you think sounds better. When adjusted for volume matching they are likely to be very close. If you hear no difference in sound quality, use whichever is more convenient.