Good question. In short, no, funding didn't change things for the worse.

In fact, for a while, it made things better - when the investor took a more active role in the day-to-day of the business, he provided a much needed buffer between us. Our relationship was never necessarily bad, but the dynamic was toxic for the business, as it impeded communication. Literally having the investor (or his highly business-skilled minion) in the room helped no end. It was when they went hands off that things went really funky, as while things around us in the business had changed for the better, as well as our abilities to grow and manage it, our interpersonal relationship still had the same problems. Having an extra head in the room was a massive help, and was actually the principle reason I'd been so keen on the deal - we didn't need the cash (a whole other story over differences with him as to how to manage finances) - but only when it was there. A monthly board meeting just wasn't enough.

So - I can't say for absolute certain, but if you already have a good relationship, it's unlikely to change. Just ensure you continue to be open with one another.

Also, if they're good early stage investors, they won't dictate - they'll advise.

Thank you again for your really insightful comment and answer. This is the sort of stuff that should be front and centre in "Building A Startup 101". Co-Founder relationships often get pushed to the bottom of the list, when it should be a major factor in deciding if a business will be viable and healthy moving forward.