There's actually 2 ways I've tried.

One is to use the midhorn that Wayne has drawn up plans for. I used the same size of horn as if I used a larger driver with the speaker end adjusted a bit for the smaller speaker than the plans were originally for. This leaves you pretty close with efficiencies and the PHL1660 is suited to a horn. They even mention horn use in their specs on this speaker.

The second option I am using now is to bi-amp the speakers with the low and high frequencies on one crossover and nothing connected to the mid. The mid gets the second amp with just the low end controlled on the crossover. Probably wouldn't work with a lot of watts going through it, but I'm not filling a concert hall. I'm sure it wouldn't come out perfect if you measured it but it sounds good.

Grant.