I am also owner of XTZ 660, same year, 4BW Europe model A/CH. I have done several modifications so far as follows:
- better frons springs, stock springs are too soft
- different jets and open Arrow PD exhaust pipe
- H&B crashbars, very usefull modification according my expiriences :-)
- rebored cylinder on first oversize in stronger tolerances ( no oil consumption at all )
- always use enduro tyres T63, much better offroad handling than dual-purpose stock tyres as , for example, Siracs, Trailwings or Trailmax.
Altogether I am quite satisfied with bike on unpaved roads, way much better then in stock design.
AnteK, XTZ 660