Please see below for diagrams on how to make either a balanced or unbalanced connection from your 851N to your 851A.
Using either of these analogue connections will ensure you make use of the high quality DAC within the 851N.
Using RCA cables will enable an unbalanced analogue connection to your 851A. RCA cables are very convenient and fairly cheap, whilst still providing excellent quality.
Using XLR cables will enable a balanced analogue connection to your 851A. This will provide a cleaner signal path compared to an RCA unbalanced connection, especially over longer cable lengths. However, the increase in audio quality they provide is subjective when used over shorter lengths. Generally we would recommend a balanced XLR connection when using cables in excess of 5 metres.
Please note: If using XLR cables, repeatedly press the Source 1 or Source 2 button (depending on which ports you are using) to toggle between 'Balanced' and 'Unbalanced'.