its been a long while since i've had sam smiths nut brown, but i did drink it on maybe 10 occasions or so.
imho its probably something like 91% base malt (MO or something), then 3% chocolate, 3% brown malt, 3% medium crystal. i always found it pretty thin and while definitely somewhat flavourful not very much so.
maybe im wrong, but my recollection is a thin-bodied beer (like all sam smith stuff [i love some sam smith products!!]) with a light nutty/roasty taste and light-brown clear appearance