Just spitballing here, but I assume it is a combination of lowering the Fraction slider in TRANSLUCENCY and maybe upping the level on OPACITY until it dials in correctly. I've also cheated these things in post a lot, either by rendering the cubes seperate and comping them into the scene in post where I can control layer opacity, or by doing a separate render with and without lights visible and then adjusting to taste.
Don't know if that helps any....