Going above 44.1 or 48KHz sample rates really doesn't do much unless you have files and players outputting that.. Regardless I still crank mine up to 96...placebos ya know...lol.
I'm not uber familiar with the Vali but are you using optical or 3.5mm to RCA?
Creative does have that sound signature you describe...and I believe a lot of that is in their software.
Also you're absolutely right on your sound card being a DAC for you...that's exactly what its doing. So does your onboard audio when you're using it for audio output source.
As said before go with what sounds the best and enjoy.
Those are good headphones you have too. Have you thought about trying the K7XX? I almost did...but seeing that I had to retire my beloved HE-400s because of 2-way noise pollution...I couldn't justify another open pair to collect dust. So I grabbed the K553s and absolutely love em. No they're not as fast, wide, separated or bassy as my HE's but damn are they close enough for me to thoroughly enjoy. I'm able to crank the bass and still maintain the level of detail and clarity I've been spoiled with.
Anyways back to your onboard vs dedicated sound card...the card is more powerful and has more dedicated hardware so it should sound better if your drivers are good...and it sounds like they are. Pun not intended.
So stick with what you have and be happy for now. You have a good setup and while not having that sound card would simplify things if you can't enjoy your audio...sounds like a fair trade for keeping the card installed. Enjoy!