I'd like more color on the low frequency side, I find most sound cards have quite clear highs (highs are pleasing always) but the problem I find with most audio is muddy/washed out lows. Mids are also hard to come by, since they're technically the most repetitive sounds, being the vocals and fills, I like to turn that down so it doesn't sound like there's too much going on.
You're not going to get to where you want by changing op-amps. Even if the differences do exist they will most likely show up as subtle difference in imaging and sound stage which would require some pretty good equipment to begin with to detect. I wouldn't be spending $30/opamp.
Most sound cards are pretty good if it fairly recent at a pretty smooth frequency response. What better cards give you are better signal processing (DACs), better signal to noise... etc. You washed out muddy lows are probably your speakers - and possibly it's integrated amp. PC speaker manufacturers put their drivers in plastic boxes that are way too small which gives them a peak in the upper bass/midbass. This makes them sound 'bigger' than what they are and gives the impression they have a lot of bass for their size, Bose plays this game too. But all that free bass is typically mush. Plus because these drivers are in too small boxes they can't produce sound low enough to meet up with a subwoofer at a reasonable frequency, below 100Hz is a ROT. So the Subwoofer ends up playing higher than it should to maintain a flat frequency response and since it's usually stuck in a corner or under the desk, somewhere not ideal to integrate with the mains.
Usually these taboos go unnoticed for HT... because really who watches a movie and says "Hey that's not quite what a car blowing up sounds like". But it will stand out much more for music.
Errr... sorry to carry on. If there's anything else you'd like to know, let me know.