The proposed crossover layout and parts are illustrated above.
The Ceramic resistors have been upgraded to Mills wire wound and from 5W to 12W.
The Chinese MKP caps have been replaced with Ansar Supersound MKP.
The Iron core coil has been replaced by a Jantzen Air Core.
The values are unaltered, but the better quality parts are closer to the required figures.
The Bi wire facility has been removed and instead, a Star Grounding scheme has been applied.
These basic upgrades wont create a night and day difference in sound quality, which was already very good. There was however, a slight coarseness/grain audible in the mids/lower treble, the new parts will hopefully remove this.
I suspect the ceramic resistors and cheap caps in the signal path were the main cause of this.

I already have a pair of 1R5 mills resistors and a pair of 6.8uF Ansar Supersound's from my parts bin, these will help reduce the cost of the upgrade.