I tried the mini to RCA route and I had the same problem. Then, I bought a sound card with a digital output and the db level and sound quality improved dramatically. There's also no hum.

Eventually, I'm going to try an ethernet/optical adapter and bypass the sound card.

Here's a question for you, why not just split the SPDIF signal?

Sound quality from the analog of the sound card really doesn't hold a candle to the digital...at least that is what I've found.

I run my receiver off the SPDIF out of the computer.

And, why not give your creative speakers the analog? Or, better yet, amp your creative speakers OFF of your receiver, and just have an SPDIF signal going to the sub for its frequencies...