why not just use AsRock H81 Pro BTC? built for mining. I have it, and it works without problems under ubuntu 13.10
For Intel there are a few. For amd not so many.
Do you have x1 risers connected into your x16 slots to run 6?
More to the point, is it x1 usb risers or x16 to x16 ribbons?
I have considered the AsRock H81 Pro BTC because its so cheap but its only for Intel. I try the whole day today to get the MSI Z87-G55 to work with 7 GPUs under Linux but I failed

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I prefer AMD because I want to support AMD CPU. And I want to build a small CPU farm too, just 20 AMD CPUs. Not so big but its fun.
Btw, thanks anyone for recommendation. G9.
before I bought the H81, I also wanted to go with a AMD cpu. However, the price of the H81 is perfect, it works good and offers a lot. The whole rig (celeron cpu & 5 x 750Ti) only pulls 240Watt from the wall (according to my kill-a-watt), so maybe you want to think about going with AMD again. If you build 20 rigs, you can save A LOT of money (just by going with the cheap H81)!