To follow up on my post, here's the rest of the story...
I added baking soda to the keg, in the vicinity of 20 - 30 ppm, and viola!!! The undesirable tart/tangy flavor vanished. It is now the beer I wanted to enjoy. Also, after amending the beer I detect no unpleasant flavor resulting from the addition of bicarb - the sodium level is still well within reasonable range, lending no salty element to the flavor. Thanks to all who offered input on this.