Another great option is Martinelli's 17" horn. Since it's a little larger, it can be used at a lower frequency. It will work just as well up high and looks very attractive with the Professional Series seven π speaker because the width of the horn matches the width of the cabinet.

Stage Series seven π loudspeaker at MAF 2003

The speaker shown above is a Stage Series seven π loudspeaker in the hotel room at the Midwest Audiofest. The Professional Series seven π uses the same size cabinet, so they look the same.