My first thought was collimation to. Soldant, are you sure that you alligned the mirrors correctly? Have you used some sort of tool?
I've also looked on the site and noticed that the only 6" scope they is a 150mm with focal length 750mm making it an f5. And the mirror is said to be a parabolic one.
One more thing, this scope is not as bad as you think. I own the
4.5" version (mine is from Spectron) but trust me, Optisan scope = Spectron scope
. I've had a lot of great times with it. Nice views of the planets, couple of DSOs. One thing I've should have done in the beginning was buying new EPs. Mine were made out of plastic optics (same for the 2x barlow and 1.5x Image corrector). So try to get your collimation good, and if that works, by yourself 2 new EP's (made out of glass and with coatings) and a decent barlow.