I would cross them as low as your main speakers permit. I run my main speakers full range and use the subs to augment the bass. When I am using my Gamut L5s or Spendor S 100s I cross the subs at under 30 Hz; for my Spendor SP1s about 32. Some subs are intended to be used close to the corners of the room; my RELs are. A good CD to get to place them is the soundtrack from "Sneakers"; the set up guide for RELs on the Sumiko web site is also useful for setting other subs up.
