This information is readily available from the NAD website. The C568 datasheet says that it uses the Cirrus Logic WM8741 DAC chip, which has a maximum sample-rate of 192kHz.
Like any Delta-Sigma DAC, it internally upsamples any PCM input to the maximum (192kHz or 176.4kHz) before feeding it to the Delta-Sigma modulator. Pages 54-58 of the datasheet contain graphs which should give you a pretty good idea of how good the reconstruction filters of the WM8741 are (and hence whether it’s worthwhile “bypassing” them by doing the upsampling in Roon).
Edit: Umh … maybe you mean the C658, not the C568? As @Fernando_Pereira says, that uses an ESS SABRE DAC chip (the ES9028PRO), with a maximum sample rate of 384kHz.
It uses an ESS Sabre delta-sigma chip. Those oversample a lot, at least 8x in a first stage, and then again to reduce jitter before the delta-sigma modulator.
Your ears should be the guide. I’d start with no Roon oversampling. If you feel something is wanting, then play with Roon oversampling.
The reality of current delta-sigma DACs is that there’s so much going on in the higher-end ESS or AKM chips that external digital filtering will not by itself bypass the chip’s digital filters. These chips typically have a filter bypass mode for DSD, but whether that is made available by the DAC is a different story. Even if you can use it, you’d need a lot of external processing and the right processing software (such as HQplayer) to produce high-rate DSD from typical PCM sources.
To be honest, typical consumer-level DACs like the C568 are not the right vehicle for experimentation with different digital filters, as they have a lot of design choices hardwired in. They either sound good to you, or they don’t. To go down the filter/sample rate rabbit hole, one really needs pro gear or exotic NOS DACs that offer the right level of control. Unless this is of major import to you, I’d just ignore it for now.
The 568 is not a pre-amp DAC combo and it has not networking capability. I assume you have the model number mixed up with the 658 which is a pre-amp DAC. Looking at the inputs of the pre-amp and its inputs I do not see an option for an USB-DAC input or that it is Roon ready. How will you stream Roon up-sampled material to the NAD? Maybe I overlooked the elephant in the room.
This is my understanding. If the 658 is truly Roon Ready it will not allow you to override the sampling frequency it reports back to the Roon core as acceptable. My Cambridge 851 up-samples internally to 384 kHz , but accepts only 192/24 as maximum input resolution through all inputs.