I went to buy the HD500X yesterday, took my Godin Exit22 and some Bose headphones to try it out first. Unfortunately I had this problem as well, all patches had a noticeable buzz/hum in the background. We tried a different guitar, but it was just the same, a better quality lead helped reduce it to a degree but it was still there. The guy in the shop thought it might have been a power supply issue, but we couldn't resolve it so unfortunately I had to walk away. They only had one unit in stock, so I'll give them a call when they get another in to see if it's the same.