ALC 883 Onboard SPDIF vs Standalone DVD Player

I want to make my PC into a Home Cinema Base. One of the issues is to get the AC3 sound out of it. What I did was to use the SPDIF onboard output. However, I did some listening, and it turns out the sound from the PC SPDIF is quite different from the standalone DVD player's SPDIF (it's a good quality DENON). The DVD player sound is much more datailed and more clear. PC sounds a little as if heard from 15 more feet away, strong bass, weaker mid and high range, much more 'blurry' in general.

WHY?

My first thought: "It's the onboard ALC 883. It's weak." But as I read, if you use the SPDIF output, your soundcard is in a 'passthrough mode', not involved in the processing. As someone put it, 'digital is digital' and one soundcard should not be different from another in this respect. It transmits unprocessed bits into the reciever (amplifier), which then precesses the data.

Second thought: "Player software". So I did some blind listening tests and couldn't tell various audio players apart. They all sounded the same, and all different from the standalone DENON.

I checked that the SPDIF sound control in windows volume is GREYED out, meaning the soundcard should not be reverbing and the sound should be unprocessed (correct?).

I don't know what else there is I'm missing. I'm even using the very same coaxial cable to plug both devices into the Yamaha Amplituner.
Can it be that the DENON so much high-end that even its SPDIF sounds better? Or maybe it's some neat trick, some equalizing it does I could mimic in a PC player (though I did try a bit of this and to little effect, deffo not compareable)? Or am I doing something wrong?

Do you guys thing a hight-end sound card could help? I ONLY intend to use the SPDIF output. I just want it to sound as good as it can.

Thx for reading and thx in advance for any help