Only for the AX series are Seasonic now. Used to be the HX too, but now it's only the 650W that is.
As for Haswell: ANY PSU with DC to DC will support Haswell's low power C7 state. Even if you end up with a cheaper power supply that uses group regulation (i.e. Corsair's CX Series), the voltatges only seem to go out of spec when there's no load on the +12V >> AND << the loads on the +3.3V and/or +5V are very high (for example, I tested a CX600 and the +12V didn't go out of spec with no load on the +12V, but did once I got the +3.3V and +5V up over 5.6A each). This is a highly unlikely scenario. IF it does happen, the only issue is that OVP (over voltage protection) prevents the PSU from coming out of sleep state. If that does occur, you should be able to disable C7 sleep state in the motherboard BIOS.