Personally, I think that before you mess with the azimuth setting, you should clean and demagnetize the heads first. Both of those conditions could result in muffled sound with poor high frequency response. Being that the azimuth really shouldn't change significantly over time once set properly and that dirty and magnetized heads is a far far far far more likely condition that occurs over time to all magenetic tape heads than the head alignments changing over time, I'm of the opinion that the azimuth setting should not be messed with except as a last resort since the proper and best way to adjust them is with an oscilloscope in order to check the phase alignment.