You have what is called "gain hiss" This is caused from improperly set gains. Gains on the amps up too high. When running a high voltage line driver like the 420. You will want to re set the gains on the amps. I wrote a set up guide on our site. On the 420 page called settings & installation. Take a look at this and follow the guide to re tune the system.
You want to be able to turn your 420 up to almost all the way and the radio to about 85% of max and that should be your max volume level. If you can only turn the 420 to half way. It sounds like you have the gains on the amps up too high.
Best thing to do is write down where they are now just in case. So you can always go back to this point. But if you follow the set up. You will be able to get a louder and cleaner set up