Unfortunately the 'better' PSUs have overload protection that will be triggered by a cube. It sucks I know, my 1500W is crying in the corner because its relegated to powering gen 1 Avalons.
The thing about the "ATX" PSUs is that they are purpose build to power a computer, a Cube is not a computer and has different power requirements; surprise, there are problems. They are being used because they are inexpensive, they are mass produced and they are cheap.
In the "day" when I was designing electronics I was used to purchasing single voltage units with current foldback for over current and crowbars to prevent over voltage. They can still be purchased, Lambda is still in business. But the price is much higher.
But today one can buy a 12v 30a supply from Amazon for $25 to $40 +shipping, just search for "12v 30A power supply". They do not come with PCI-E cables & connectors.
Example: http://www.amazon.com/dp/B00D7CWSCG/
Some one should try one.