I NEVER got my single Cube running properly on a CX750. It works perfectly on a cheap OCZ 600W, though.
That's odd, the specs on that PSU are more than enough for a single cube, whereas the specs on the OCZ 600W, you'd be pushing it if you tried to overclock (separate 12V rails @ 25A each). Not the first time I've seen people post about issues with that particular Corsair model though. Maybe you just had a sub-par CX750? Weird.
Yeah, I agree it makes no sense. The CX750 is powering everything else fine (2x Blades, 2x Jalapenos, 2x USB hubs, fans, etc).
The OCZ had been running fine with the Cube overclocked - even considering it's running off a single rail (the two PCI-E cables run off a single rail).