It's unclear whether your source, NAD amplifier or both are the cause of the graininess. I suspect it's a bit of both.
I definitely wouldn't go the route of a new preamp. The Cambridge 640C is fairly well regarded. I like the recommendation of the Prologue 2 integrated amp. Perhaps a good plan would be to purchase a used Prologue 2, try it in your system and see if the change results in some improvement. If not, then sell the Prologue 2 and investigate a new CD player, but I have a hunch the Prologue 2 will be an improvement.
I definitely wouldn't go the route of a new preamp. The Cambridge 640C is fairly well regarded. I like the recommendation of the Prologue 2 integrated amp. Perhaps a good plan would be to purchase a used Prologue 2, try it in your system and see if the change results in some improvement. If not, then sell the Prologue 2 and investigate a new CD player, but I have a hunch the Prologue 2 will be an improvement.