Uninstall the Sound Blaster MP3+ from the Device Manager. Power down. Plug soundcard in to the USB hub. Power up. Windows will load the drivers again. Think it has just gotten in to the habit of looking at a specific USB port for the device. If it is plugged in to another port, Windows will get ****ed off and does not run the device. I've noticed this with a few memory sticks, my external HDD, and my printer when I use them on my laptop. They have to be plugged it to the specific port in order to run. When I set up my CarPC for testing, everything is running through the USB hub from Day 1.
As far as your popping sound goes, here's what's up. The amp turns on. But then Windows initialized the sound card causing the pop. You need to delay the time that the amps are turned on until after the soundcard is initialized. Search the forums for the words amp and delay to set you on the right path. More than likely you will need to make use of a delayed timer relay.