lens recommendation - How to determine which lenses are good at testing teleconverter quality?

I have a Kenko 3x teleconverter and am getting generally soft images (using a Canon 5d Mark II on a tripod).

I am trying to find out how much of this is due to the quality of the teleconverter and how much is due to the fact that it magnifies the flaws of the used lenses so much.

Is there any database of tested lenses (preferably that includes vintage glass so I can afford them) that I can use to figure out which lenses to use for such testing? I am thinking that lens sharpness should be the key here. (I am thinking telephoto primes 200mm and up, preferably on the longer end of that if I can find something affordable enough)