A friend built a pair using the D series 8" woof, the F 6" mid and the RT2 planar tweet. He went with 3rd order x-overs using alpha core inductors and solen caps. Turned out very well. The drivers are very well integraded and I think if you dropped the extra coin for higher quality caps such as maybe auricaps they would sound even smoother. Can't remember where the mid takes over, but the high pass x-over is 1.9k.