Good examples of communities that have managed to maintain quality well are /r/askhistorians and lobste.rs. Ask Historians managed to do it by ruling with an iron fist. Lobste.rs by heavily curating which members are able to join. Another interesting example of retaining long term quality is visible in private torrent sites- many of which employ both an invite system and strict rules.
Finally, what made Hacker News so good ~4 years ago was it was originally populated by people within the industry with decades of experience. You yourself said this was for the financial industry-- trying to pick up early adopters here makes about as much sense as trying to pick up adopters for a medical industry community on an economist forum.
[1] /r/trueaskreddit, /r/truereddit/, /r/truerreddit
[2] https://en.wikipedia.org/wiki/Technology_adoption_lifecycle