I've read about seasonic x series PSUs apparantly they are completely passive up at up to 25% load.

My current setup draws 180w from the mains on idle, so according to that I should 720w at a minimum.

I'm trying to be economical too. So I was wonder how strict is this 25% rule. And would the 660w version run passive on my rig.

Thanks,

Wow! Did you mean load power is 180W AC?

If so, that's:

162W DC @ 90% efficiency.
153W DC @ 85% efficiency.

Take a look at the number of low noise PSU's in this table at your DC load power.

Don't worry about passive - just pick one that is below your noise floor.