DPA 406x / CS-HEB > batt box > M1 is a kick-ass stealth setup. Upgrading the mics will have the single biggest impact on your sound. Unfortunately, in the strictly stealth arena, there aren't alot of options close to the DPA 406x / CS-HEB price point.
First things first: What's your budget? And what, exactly, would you like to improve in your sound? Or, put the opposite way, what about your recordings don't you like?