Teleconverters are just sophisticated magnifying lenses and as well as introducing their own faults multiply the faults in the main lens. With stacked converters you are adding more glass air interfaces so one 2x converter would be better than stacking two 1.4's. A recent magazine article I read concluded that a high quality converter was slightly better than just cropping on the computer but I imagine that stacked converters would definitely be worse. I would stick to the 1.4 and possibly buy a 2x for the extra reach as well but I would not stack them.
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