help with headphones and Blu-ray audio on a HTPC

I've built a small form factor mini-ITX system that doubles as a LAN rig and a HTPC. I would like to be able to listen to the native DTS-HD MA and Dolby TrueHD tracks on my Blu-ray discs in the purest form possible. I mostly watch movies using a pair of Sennheiser HD598s. How to get the best quality audio for movies with them?

I was thinking I would need to bitstream the audio and video from the GPU (GTX 580) over HDMI to a receiver and then out to a headphone amp and then to the headphones. I'm a pretty lost when it comes to high quality audio. I only bought the Sennheisers a month ago after spending years using $30 headphones. On my main desktop I have an Asus Xonar STX that works as a high quality sound card and headphone amp but I don't have that luxury on the HTPC's mini-ITX board.

Right now I just have the HDMI running to a 46" Sony XBR6 HDTV and there's no way for me to get the HDMI audio to the headphones. I bought a stereo RCA to 3.5mm adapter and connected it to the TV's audio out ports but I don't get any sound. I assume this is because the headphones need a power source, which would be provided by an external amp, correct? Any recommendations for an amp under $200? The 598s are nice but they aren't that high end so I don't need a super expensive amp either. 

Is there any benefit with adding a receiver to the mix? I had though I needed one to do the DTS-HD MA and Dolby TrueHD decoding but again, I don't know for sure. I've got all the software support I need, Cyberlink, Arcsoft, etc. Thanks in advance.

What receiver are you using?

I did some Google searching earlier and people seem to be torn between headphone amps and receivers. Some say a receiver won't provide very good headphone output by itself, some say receivers are fine, and some say it really depends on the receiver.

I'd be willing to spend up to $300 on a receiver and $100 on a headphone amp if it was worth it. A $300 receiver won't be great I know, but I'm not planning on hooking up speakers because I watch movies in the evening and can't have speakers very loud in my apartment. If a $300 receiver will suit my needs then I'll drop the headphone amp and just use the receiver. Similarly if the $100 headphone amp won't add anything, or is worse than the receiver's headphone out, then i'll stick with the receiver option.