I would upgrade your spotter, those Nikon Monarchs are better glass than they get credit for. I have a pair and I use them just as much if not more than my Ziess Conquest HD's. The 8x42 Monarchs are a lot lighter and do better in low light due to the objective(?) I would also agree that the jump from 8x42 to 10x42 is not huge, in my case not always worth the extra weight. The 8x42's will do better both in low and still hunting. I like my 10x42's for scouting when I'm not carrying a weapon or when I'm hunting really big country. But even then, the spotter on 15x works better for really long distances.
Save a few hundred more and get a Razor spotter, it will increase your glassing efficiency by a lot more. I jumped up to the Viper last season and use my binoculars even less now. I scan on 15x through the spotter all the time. The trick to it is to learn to open both eyes when you look through the spotter and it eliminates nearly all the eye fatigue.
In my experience if your spotter is of lower quality than your binoculars, it will leave a lot to be desired in the field. Especially when judging animals from a distance.