Presumably you have visited the spotify community forums @ spotify.com? I would start there.
I'm having a party tonight and i'm currently setting up the sound system in my living room for the loudest music possible. I have some problems however...when i'm playing music on Spotify, the green volume bar in Windows volumecontrol doesn't hit the max or even 80%, which is the actual volume output.
What i mean is, i've pulled the arrow all the way up to 100, but the green bar that moves when music is playing only hits to 50-60%, which is way too low and complicates things. So when I also raise the volume on my sound system, the bass is going wild at max volume, but the volume just isn't at the same level.
On Chrome browser however, on soundcloud for an example, the green volume bar ACTUALLY hits 90% circa, which makes for great music volume in the living room. So what the **** is up with Spotify?