My approach is different from your approach, but i have decided to add anyways under "what would you do?".
I would not get the NAD if i did not like it. I would get the Arcam if I did. Monitors just do not need that much power, especially when quality is compromised. I also do not understand biamping a ~$700 monitor. It's just more of the same, but not really more. Because you like seem to really like the Arcam, I am not thirlled by adding this but a used Classe CAP-100 or 80 (both rated at 100w) is in your budget.
And BTW-FYI-the 105w rating of the C370 is very conservative.
Good luck, hope you have fun buying and listening.