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So I currently have a TC-14 1.4x TC on an older Nikon 300mm f4 AF lens.
I presume if I want a 2x teleconverter I'd get a TC-20 and be done with it. Optically is that much worse than stacking two 1.4x and related, is using less teleconverters, if you had to use it, a lot better or are results similar?
Effectively I know that adding a TC reduces quality. Is the quality loss dependent on the magnification or more on the amount of glass between the lens and sensor?
There are no absolutes when comparing disparate optical designs. It always comes down to the actual performance of each system.
It's quite possible that two very high quality 1.4X teleconverters could outperform a single mediocre or downright bad 2.0X teleconverter. But I wouldn't bet anything on it. If all three converters have roughly the same optical quality then a single 2.0X should beat the dual 1.4X teleconverters fairly easily.
Is the quality loss dependent on the magnification or more on the amount of glass between the lens and sensor?
It's some of both. The magnification of the flaws in the original lens will be basically the same either way. The additional glass of two 1.4X converters with no additional corrective benefit will normally degrade the optical quality more than a single 2.0X converter.
