Should I buy average tires with puncture resistant tire liners or just puncture resistant tires?
Exact center of the outside of the tube (and no debris) indicates a pinch flat.  Why do you assert not a pinch flat?

My experience with tire liners is they abrade the tube.  I would wear out a tube a year riding about 500 miles.  

Puncture resistance tires work better for me.

Why you are not seeing debris in the tire is a mystery to me.

You might also try tubes with sealant for small punctures.