Isolation transformers output voltage higher than wall

What are you measuring with? There could be as much as a five to six volt difference between true rms multimeters and averaging multimeters when measuring wall voltage. All multimeters are accurate with a clean sinewave power but not with wall voltage.

It's a small compact plug-in meter with a display. My wall voltage does not drift much. It typically is 121-122 volts.

I've seen those cheep plugins that were accurate and those that were not.  

Anyway, you've got nothing to worry about.  Other than your transformer may be doing more harm that good.  

Jerry

@carlsbad I always listen to music with which I am very familiar and if I sense the sound quality is compromised in any way then I won't use them.