I thought this might be helpful as some use % and others use -db.
Manufacturers sometimes list the total harmonic distortion (HD) in
percent, but not how many db down the total harmonic distortion is.
(This is not a listing of each individual harmonic level, but the
predominant harmonic should not exceed the total harmonic figure.)
Others, usually tubers, sometimes list in specific db down, but don't convert to
percent HD.
Here is a link that one can calculate db down from the HD figure and
visa versa.
http://www.sengpielaudio.com/calculator-thdI.e. If the distortion of a gainstage or whole component is .5%, the
predominant harmonic (distortion harmonic) is approx 46 db down.
For 0.4%, the figure is 48 db down.
If a preamplifier has 0.1% HD, the predominant harmonic is approx
60 db down.
If 0.01%, 80 db down.
Of course there are other distortions, such as intermodulation distortion.
I hope this makes things easier when comparing what info is provided.
Happy listening.
Steve