Tom,
I used the X1 with 3 different p.a. systems just about every night and never had a noise. It was extremely quiet. Be sure everything is grounded the same and preferably plugged into the same a.c. outlet. Often, the outlets at different buildings are incorrectly wired or not grounded. That will usually cause a hum or buzz. And you might want to try different cables between the line outs and amp. Older cables can start picking up noise over time.
With the Yamaha PSR8000 I experienced noise when the midi cable was hooked to certain devices. I ever did get it completely out, despite changing cables, outlets etc.
Also, you might try using the X1 with a different amp to see if perhaps the problem is in the amp.
If it does need work, I will help see if the warranty will cover it.
Don