I haven't posted much music on SoundCloud but when I have, I thought it sounded really bad. There is no getting around the fact the music is going to get converted to a lossy streaming format but I know I've heard stuff on SoundCloud that wasn't horrible. After doing some research, I found out SoundCloud's conversion algorithm adds 2.7 dB of gain... So, if you're doing DIY mastering, I suggest mastering as though you're going to distribute CDDA or PCM but create a second version with the peak limiter set to -3 dBFS for uploading to SoundCloud. I tried it today and it makes a noticeable difference. If your stuff is already mastered, you can adjust the gain in the audio software of your choice before uploading to SoundCloud. And yeah, I know, I need to get with the program and use Bandcamp or Topspin...but I hope this helps.