Black trace in Fig 2 is the response of the D220Ti/PH715
combo with the Synergy crossover at 1,000Hz.
appears the combo can in fact be crossed as low as 1,200Hz.
With the resonance peak at 680Hz, it is likely that a conjugate
network would be necessary.
Red trace is with the Synergy at 1,750Hz. At this frequency,
there's no need for the conjugate network to damp the resonance.
Power handling is obviously improved too.
the Violet trace is with a 8.2uF capacitor inserted in series
with the combo. This corrects the dipping response of the
2 - D220Ti/PH715 Frequency Response