rkk1, are you using a laptop? Are you coming out of the headphone out?
The reason I ask is that sometimes that can cause problems. If I use my laptop, I always use an external USB soundcard with RCA outputs to the mixer. This goes a long way to cleaning up the sound. Just one example
http://www.amazon.com/IKEY-AUDIO-ICO.../dp/B002MFBVGI I use a similar item from soundblaster. Some mixers even have a USB input with a sound card in them.
Then all you have to worry about is the ground hum from the amp. The isolator you ordered will more than likely take care of that. As far as the "fade" you were talking about. The venu may have some power issues. If the service is already overloaded (most are) and you have, for instance, a cooling compressor going out, it will even dim the lights when it kicks on. In that case, a power conditioner or an uninterruptable power suppy (UPS) will be necessary to protect your equipment.