Hello X1 user,
I am a Solton dealer in the Baltimore Maryland area. I recently demo'd the X1 for Solton at the Los Angelos NAMM show.I have never had a problem with the X1's headphone jack. I have plugged into many a X1's with headphones. None of the Solton owners have mentioned a problem as well.
You can certainly try different headphones . Or have the keyboard evaluated by the Local Technician. Good Luck.......oneils4@home.com
Dan O. Guitar Center Towson