HTPC as pre-amp out to multi-channel amp

Get your 3.5mm Y splitters and go for it. I'm running my Denon AVR the same way using the analog multichannel inputs.

But by plugging those into the line inputs of your Denon, you are still using the pre-amp of the Denon.

If you were to plug those directly into the line inputs of a power amp without an intervening pre-amp, I'd be worried about overloading those inputs. I don't know if the line levels are compatible or not. I think the line outputs of most sound cards are made for powered computer speakers, not for a hi-fi power amp.

I output directly to my amplifiers. I use an Asus Essence ST with the H6 daughtercard for 7.1 output. It actually has RCA outputs for all 7 channels and has excellent sound. I like the Asus sound cards because you can use the Asus XONAR unified drivers for bit matching with ASIO output. You might also consider an HT Omega card or the Auzutech X-Meridian 7.1 2G. I don't use any of the card's features. I just use it as an internal 7.1 DAC.

To listen to music or play DVD/Blu-ray I use J. River Media Center 16 (MC16). It decodes pretty much everything (even DTS-HD with the dtsdecoderdll.dll file from a trial ArcSoft installation). To play Blu-ray you do need something like AnyDVD HD running in the background and you don't yet get menus. MC16 provides all the receiver functions like advanced bass management, delay and distance settings, advanced matrixing similar to Dolby Prologic IIx, and volume normalization. It also has more advanced features like its VideoClock (syncs the audio to the video so you have no dropped frames), automatic display resolution switching, Parametric EQ (apply as many EQ filters as you need down to 1 Hz), Volume Protection (helps prevent an unintended full signal accidently going directly to your amp), and advanced speaker routing. For example, if you have a 5.1 speaker system with two subs and a 7.1 soundcard, you can copy the LFE channel to an unused channel for the second subwoofer. You can then apply separate distance and EQ settings to each sub before EQing them together.

I also have a power amplifier connected directly to my HTPC. I've had good luck with the Creative X-Fi cards. I'm using J. River Media Center to manage all the decoding and processing. My latest project is to use the same power amplifier to bi-amp a subwoofer and perform calibraration (Linkwitz Transform, etc.), all from inside MC16.

desertdome also gave good hardware advice. He's a smart guy about this stuff.

does no one else use vlc?

Before you spend money on an expensive sound card, I would suggest you try out the motherboard's 7.1 ch analog outs. Nearly every board nowadays has 7.1 output, even the cheap ones.
Try it out and see how you like the sound quality. Unless you are an audiophile with exacting standards, you will probably be happy with the onboard audio quality.
And TMT and PDVD will both decode HD audio formats to analog line out.
Just look for a board with a supported audio controller.

I was thinking of this thread today when I cam across this:

http://cgi.ebay.com/HTPC-Custom-Made...ht_1232wt_1141

Doesn't exactly help you with the sound card, but this I think is really, really cool if your were to go that route.

How do you do this - where are the settings that makes this posible?

FFDSHOW can remap channels.

Using a HDAV 1.3 I got on ebay for this and am happy with it.