What you are looking is for a 'product engineering team' as against a generic 'web development company'. From the stacks you've mentioned, it looks like you have several moving parts (Existing code - Both ASP.NET and Opensource, both which can be extended/reused/refactored, Integration with some enterprises systems where knowledge of fullfilment/order managment/enterprise procurement APIs would help). You also need a team that will perhaps have to write APIs that do two way communication with external systems that your clients may use. While doing all these, the team needs to add new features, make architectural decisions that should not impact the scalability (ie carry less technical debt).
All of these are classical strong point of good product engineering teams and not web dev shops that can do website and some dynamic pages.
Bad news first: We are not a dev shop.
Good news: We are a special kinda firm that discovers dev shops for product startups (we discover the good shops based on feedback we gather from their customers, over a call). We have some 50 such shops that we would call as elite ones. May be we can put you on to a handful that would get your context (and offer peaceful night sleeps to your CTO).
PM me or just write to firstname.lastname@example.org
. See you!