There's a couple of things you can try to address the noise in your signal. It sounds like a ground issue to me.
1. Try pressing the ground switch on the back of the Synth.
2. Make sure your Computer, E-MU and Synth are all sharing the same A/C circuit. Having them all on one power bar, for example, can help.
I find that having USB connected along with audio devices can be especially tricky. For example, I run my iPod into an audio interface at the office. When I also connect the USB cable to the iPod to charge it, there's a horrid noise, even though everything is grounded properly. It's just the nature of all of the electrical components causing interference with each other etc. and is sometimes unavoidable.
To answer your second question... yes and no.
The Synth can't be used as an audio interface, so you won't be able to use it via USB to process a signal from your DAW. However, you can use the Synth as a signal processor using an audio interface/mixer.
You'd have to set up an "Aux Send" in your DAW and route the output of that send to a physical output on your I/O. Then send from that output to your Synth and from the Synth back into your I/O.
Your ability to monitor/route everything will be dependent on the number of ins and outs on your specific I/O unit.
If you want a device that will act as an audio interface directly connecting to your DAW, you'd have to use a VoiceLive 2 or VoiceLive Touch.
I hope that helps!