HD558's are in a completely different league compared to Creative's, absolutely no comparison, and if 558's dont sound any better than creatives, then the problem lies somewhere else, not in the headphones themselves.
1) What kind of source do you have? If you use them with onboard sound card on PC, dont expect good sound.
2) What kind of music format do you use? You wont really notice a difference if you listen to 128 kbps MP3's trough winamp. Use foobar2000 or something similar and get some properly good sounding records.... like
http://avaxhome.ws/hraphile/Various_Artists_Blue_Coast_Collection_The_ESE_Sessions_24_96.html Buy it, of course.
Also, keep your equalizer settings (if you use them) to default.
3) How long have you used them? 558's needs well over 200 hours of burn in, and then they start to mellow down and become darker and smoother, also, you need to give your ears and brain time to adjust to the sound. DON'T force yourself to listen at high volumes. Mine have 500 hours of burn-in, and they sound completely different compared to new 558's.
4) Have you modded them? Removing the foam inside (google for 558 / 555 mod) will make the bass considerably stronger.
5) You DO need a proper amp. If you think the only thing an amp will do is make your headphones louder, then you're very wrong. You said they sound flat on low volumes. Well thats because you dont put enough power into them.
Have a look at this graph:
You see how much more power they need from 40 Hz to about 150Hz (bass area) ? Impedance jumps from 50 ohms to almost 300 ohms. Thats why you're getting no bass.
NOTE: if you check any other headphones in this price range , you will see that none of them have such spikes in impedance vs. frequency graphs, instead their graphs are flat. Why that is...I dont know, but its obvious 558's like power to shine in low-freq area.
Buy an entry level head amp....like Fiio E9 or Pro-ject head box, and you will hear a huge difference in bass. Then buy a proper sound card or DAC, and you will hear an even bigger difference in clarity, smoothness and soundstage. Dont get fooled by their low impedance on paper and dont believe those who say "they're easy to run".
Yes, they will go pretty loud without an amp, but they wont sound nearly as good as they do with a proper amp.
Read my review on HD558's and you'll see I felt the same as you, until i realized i was getting 50% of their potential. Now, after about 9 months, I absolutely love them. They're very comfortable, bass is just right, mids are full and mellow, and highs are smooth and crystal clear. Very nice to listen, very addictive sound signature.
My friend recently bought new 558's because he liked mine, and mine ((flac's > foobar2000 > asus xonar stx > fiio e9 > headphones) sound MUCH better than his.
However, even after the mod, DON'T expect subwoofer style bass, because you'll never get that from open headphones. You'll only get natural sounding bass, which will never be overpowering, but it will always be present and satisfying.
