9 speed is 9 speed, the 'super narrow' refers to 9 speed chains, as in '9-speed super narrow chain such as CN-YM81 / 7701 / HG93' quoted from here http://techdocs.shimano.com/media/te...9830686193.pdf the 7700 cassette is 3 generations old, when 9 speed was new, and so were narrower chains.

The KMC X9 SL should be fine, when you removed the rear wheel to install the cassette, was the wheel re-installed correctly?