The DS and DX use the same audio processor the CMI8788.
Both the DS and DX do not come with a dedicated headphone amplifier.
So I'm going to guess they will be very close, if not matched, when powering headphones.
The DX would be the best card for analog 5.1 speakers, also comes with Dolby Digital 5.1.
You can upgrade the op-amp on the DS, not sure if it affects the headphone.
The DS does not come with Dolby Digital, just DTS.
The DS (and DG) use a C-Media audio processor designed for the PCI bus, so no problem getting power from the PCI slot.
The DX (PCI-Express) uses a bridge chip to allow it's audio processor (designed for PCI) to communicate through the PCI-Express slot.
The bridge chip can not pass power from the PCI-Expess slot to the card, so the DX gets it's power directly from the power supply using the floppy power cable.
Get the Asus Xonar DG (PCI, CMI 8768), it comes with a built in (half way decent) headphone amplifier.
It can also do Dolby Digital Headphone 5.1 (like the DX)
and it's a cheap price.