SMP audio equalizer Custom values sometimes get lost

[brittney24]

Using SMP 20.4.2 (revision 9306) in Mint 18.1.
I searched this forum & didn't find hits on SMP's audio equalizer losing settings, made under the Custom preset, so it may not be common.

I use headphones, but the Headphones presets don't quite suit me.
I selected the custom preset & changed a few settings. Clicked Apply & I believe "set as default values."

It seems to keep my entered values until I select another preset - but don't click Apply / set as default values / reset, while looking at or using the other preset on some music.

When I go back to Custom, the values have changed. I believe sometimes, to the values of the most recent other preset I clicked, to have a listen.
I've not fooled w/ SMPs audio equalizer much, so don't know if this is common.

I can't find where the values entered under Custom audio preset are stored - didn't find in any smp or mplayer (which I'm using) conf or ini file. I searched the whole file system.
If I knew where it was stored, I could backup a copy to paste Custom preset values back in or maybe set the immutable attrib on that file or something.

Also, in SMP audio equalizer, are the values in each frequency column just dimensionless values? Don't appear to be +/- dB gain, as in VLC.