It seems the headphone gaming guide is invisible to you. You can use any DAC that has an optical input with whatever standard headphone amp you want. OR you can use the Playstation composite or component cable and hook up the RCA audio to a headphone amp (in this case, you'll be using the PS3's internal DAC). Just make sure to set the PS3 to 2 channel PCM unless your DAC has Dolby support (I.e. Astro Mixamp, Turtle Beach DSS, Creative Recon USB, AX720, etc).
Btw, usb amps will not work. Like Raven says, you can buy an adapter, but you still have to run from the adapter's 3.5mm out to an amp's line in. It makes more sound quality-wise to use the rear ports.