The LS3/5a is a tough match for a sub because there's a broad mid-bass hump. My general observation is:
If you cross to get a seamless hand-off, you'll likely have too much bass, octave to octave. If you reduce the subwoofer's output to preserve a good octave to octave balance, you'll have a level mismatch at the x-over point and a "raggedy" hand-off.
Maybe someone else has had success and can offer a solution, but the LS3/5a just doesn't -IME- lend itself to mating with a sub. I do seem to recall however, that one of the brands licenced to produce the design (maybe Chapman?) made a less expensive variation and a matching subwoofer/stand. Don't know if/how it worked, but might be worth researching.
Good Luck,
Marty
