SHOUTcast settings can be entered by downloading and installing the Internet Broadcast plugin from the plugin gallery on PlayIt Live. The documentation for encoder configuration can be found here: http://docs.playitsoftware.com/InternetBroadcastPlugin/encoder_configuration.html
Producers tend to compress sweepers and jingles more so they sound punchier. Currently there is no way to control the volume of individual tracks but I can understand your need for this. I would recommend dropping them into your favourite audio editor and reducing the volume using the Amplitude effect.
Thanks for expressing your interest in a dark version - this is something I am considering adding next year.
You can adjust the volume(strictly subjective loudness) of files with MP3 gain.
How and is it possible to setting shoutcast V1/V2 in PlayItLive?