THREE OR FOUR $35 MICRO-ATX MOTHERBOARDS--

You could run three or four cheap micro-ATX motherboards with three cards each and smaller power supplies.  The splitters (also called multipliers) likely will not work.  Mine does not, and you can't buy the cheap 1-to-4 card splitters anymore, I checked.  Why bother with a huge $300 PSU?  The splitters running on linked PSUs will likely get out of synchronization with the motherboard due to power fluctuation.

Little cheap motherboards cost the same as the splitters.  Get one with an H81 chipset from a good brand.  The whole idea of 6 to 8 cards to a rig is a waste, if you ask me.  A 750 watt PSU can handle three RX 470 cards.  Six RX 470 cards need a 1200-to-1600 watt PSU to run efficiently.

       --scryptr