The master volume control CV is CV58, its values range from 0 to 15. The factory default value for this is 09.
Digitrax SFX decoders have a factory default value of 09 set for CV58. This works well for 32 ohm speakers.
The decoder will reset to this value whenever you do a decoder reset of CV8=8.
Digitrax SFX decoders built for use with 8 ohm speakers should have this value changed to 4 or 5 for best operation value of 4 or 5, since the decoder may not supply enough current at high volume/drive levels.
Your results may also depend upon track voltage present.
Kato USA says that their AC4400 sounds better if the CV58 value is set lower.
CVs 140-145 can be configured to set individual sound volumes for the various components of the sound output. These can be set up for a CV8=8 factory reset to have values specified in their SDF files. This means you can download a new .spj file to your decoder with your preferred default values to replace the factory values that will be reset to 09 if you do a CV8 to 8 decoder reset with the original .spj that comes installed in the decoder.