If you're going to look at this kind of thing I reccomend a plugin like Voxengo SPAN (which is
free) So you can compare the entire spectrums of both channels in real time, and see if it's unbalanced across the board, or if it's just certain elements (or ranges) within. It's also wise to have the stereo mix available for reference so that you can try and discern what the original engineer was trying to do - most quad mixes (especially from the WEA family) were either made by the same engineers who did the stereo mix, or with a high level of faithfulness to it.