I have an Audiovox FM Modulator that I use with my Empeg. I get no interference, because it connects through the antenna jack on my stock HU of my car (I also cannot get a CD changer aux-in interface for my Subaru Outback). It sounds like the best radio station you have ever heard, basically. If you can live with that, then you'll be ok.

Here is a pic of my setup. I use a relay connected to the Empeg's amp sense line to turn on the FM mod, you'll have to hook up a hidden switch somewhere under the dash/glove compartment if you are using it with a portable.