Don't know about the Icons with the Holt, but I wear my Nixon Whips under my Maze helmet.
My trick is to have it on the backside of your head (as opposed to top). I also wear a touque under the helmet with earpads removed though.
Fits really good. Haven't had any problems.
Two other options are:
1. Just get earbuds
2. Break of the headband part and stuff them in the ear pieces. I wouldn't recommend this unless you truly want to. My friend did it with his Holt. Worked for him.