Do you mean by "far enough' as to get the drivers of the sub and the mains to physically line up with each other? And the box subs offer more flexibility somehow through the servo amps controls? And how much flexibility?
Oops, a little late with this post but I'll leave it anyway
Far enough into the room for OB subs means at least 3ft out into the room. I have dual OB subwoofers (one on each channel, also paired with 94.5 dB efficient Wedgies) in the man cave which is a small room (10' X 11' X 8'). In this room they are 3' off the front wall. In my larger (20' X 25' X 8') living room they sound best 5' off the front wall. Their primary home is the man cave (repurposed child's bedroom).
I also have a sealed servo sub in the living room system. The sealed sub can be placed right up against the wall or out into the room, whichever works best. I used to have it right against the front wall but currently have it placed directly behind the sofa.
To sum up, OB subs need to be at least 3' off the front wall. Sealed subs can be right up against the wall.
These servo subs are the fastest, cleanest and most musical subs I have ever heard. To put it in perspective, these subs and amps are the same ones used in the Serinity Super 7's and in the triple driver H-Frame configuration that were paired with the LS-X OB line source that garnered "Best Sound Cost No Object" accolades at RAMF a couple years ago.
Mike