All new machines tensions are set for poly thread at the factory. Even Berninas. The floor models have been set up by the dealer to use whatever thread the store uses. If you take one right out of the box, it's set for poly. Most machines sew fine on that setting. Changing tension is simple. Loosen or tighten as much as you need.
When my machine wouldn't sew I had the needle threaded backwards. I threading right to left instead of left to right. Drove me nuts until I realized what I was doing.