General question on PC audio set-up

I've been using my Toshiba Qosmio Notebook(NB) and a 500 gig external hardrive for playing mostly flac and shn (but some wav and mp3 files). I have been very happy with the sound quality and haven't done anything exotic in the set-up.

I use the single external stereo speaker output on the NB and use a splitter with RCA plugs connecting to an Aux input to my ARC SP-12 preamp. I cannot notice any sound quality difference between playing the digital files and a CD. Not using any special cables for speakers or interconnects other than a higher end pair between the pre and the ARC D-90 amp. Use Focal bookshelf speakers and the sound is very good.

The only question is that I've read a lot of forums where people are using D/A converters and a lot of other components for their PC systems and I'm wondering if I'm missing something. Again, I'm quite happy with the current sound and have listened to many systems including others I've had over the years but not through a PC. This one sounds as well as I can expect.