I think I had the same thing. I would get a vibration under 35mph, above it went away. I decided that it was just the "knobs" on the tire. It felt pretty close to what I would get when taking my off road bikes down the road with a more aggressive knobbie. I didn't worry about it and the K60's have around 3000 miles on them right now. When checking tread depth against new it should run out around 10,000-11,000 miles. I believe that is normal so I don't think the vibration was a problem. I bought mine new end of April beginning of May.
2009 Husqvarna SM610 (Fully swappable to TE)