The only (theoretical) way to improve the sound output further is to buy a professional audio card with pro-level D/A converters. Provided your hifi is at that level, too and the program (your music) is recorded at the same level (no MP3! ;)), you could gain some quality with that. There are "HiFi"-PCs that do it that way. Since these cards don't know DirectSound/EAX and stuff, you'll have to employ 2 soundcards in your computer if you still want to play games with it.
But Ancient Dragon is right: If your hifi is only "very good" ;), you will get more improving effect from better speakers. Maybe the cable from your soundcard (which should not sound noticeably bad) is of bad quality?
I can say (PC over Hi-Fi user too, I might ad) that your choice would be anything that is USB.
Why? Because it needs to be as far away from the PC casing as possible.
I'm not sure about your config. but I can hear EVERYTHING that is going on inside casing in the form of static noise from E-M emissions from anything that has voltage.
If I crank up the volume to ~80% on Hi Fi, I can hear CD spinning, HD "clicks", fans making continuous white noise, even graphics when scrolling! All that is being picked up, mostly by analog audio CD wire that runs between CD and Sound card (on-board). When I mute Audio CD in volume control, most of the noises mute too. Rest of the noises come from MIDI channel. And mic, if you use it.
Once I used wavelab just to see the noise that is being generated with all channels unmuted. *shivers* Scary....like aliens or something.
I agree. MP3, WMA and some other formats (if not "lossless") deliberately change the sound so it would pack better.
However, AC Audio is far from perfect too. Any audiophile would prefer vinyl over CD. Reason is D/A converter.
With digital audio, perfection of the sound depends on timing accuracy of that chip. If you pay $50 or $100 for a bulk CD ROM device, you can't expect to have very precise timing when it comes to regulating spinning speed, or converting digital record to analog signal. Vinyl has no such irregular timings with reproduction.
Long story - short, I wouldn't recommend spending more than $300 on a soundcard. It just doesn't add up. you'll still be stuck with not so high fidelity sound coming from your Hi - Fi.
Only thing that I've encountered that would raise the level of audio quality with PC's is one PCI sound card (can't remember the brand) that I've seen in newspaper's ad's. It was second-hand, and the price was $800. I was, like, WTF?. Called the guy just to ask what is he selling. He said that it was simple PCI card, 1/2 of retail price, no fancy add-ons like hardware accelerating or caching or anything, just 2 jacks for optical cable, I think 8 line-Ins and 8 line-outs. And the card itself was nothing but one big A/D-D/A converter. For $1600 D/A converter I think one can expect higher audio quality.
Math is simple there. Mo' money - mo' music.