If you look at the frequency analysis* ( comparing before and after treble boost) you can see what frequencies have been boosted …
[ * see “Analyze”, “plot spectrum” ]
As you’ve found out resampling at a higher rate doesn’t improve sound quality, it just makes the file bigger.
{ Update: listening back I think I’ve used a bit of chorus on the treble boosted version, gives a stereo effect .
You could try Steve’s pseudostereo effect plugin instead of chorus }.