[...] 750Ti without power, steel risers burn through 12+ volts[...]
Of course they do... Those risers are not made for high power cards.
The very worst card for such risers, is the 750/750TI
without external power, as the card is entirely powered from PCIe, but even with externally powered cards, those risers are known to fry quite frequently.
Were you lucky enough to not get any damage beyond the risers themselves?
The asus rock BTC h61 or h81 has 2 extra power slots.
4 pin=150W (this is standard on all motherboards)
4 pin * 2= 300 (use two seperate cables)
total of 450W
No need for powered risers with 6 750ti.
That doesn't matter in this case because not the motherboard burned but the riser itself so it will do so regardless of mobo. Unless you use cards with additional power connectors as said earlier by myagui.
Does a USB riser actually use USB protocol or is it a straight replacement for the ribbon cable?
They are just a replacement for the ribbon cable for the sole purpose of connecting the parts and it's not using USB protocol. USB (cable) is just much more relieable, durable, flexible, it's shielded, cheap and convenient because it has a connection interface so all the parts of a riser isn't just soldered together into one mess.