I keep finding that food in the slow cooker can come out a bit bland so I wouldn't worry too much about it being over powering. Things like mustard seed, cumin, coriander, paprika...come out really nice and savory in the slow cooker while dried herbs don't come out as nice. This is what I've found at least and I might have to just add more dried herbs...
I'd also saute your onions and garlic before you put them in. You could even add the tomatoes to the saute, cook until the oils separate and then put it all into the cooker. The extra 10 min prep can make a huge difference.
Patrick, don't sweat the small stuff