I have a midrange setup that I'm quite comfortable with. I have an American Strat that I bought ca. 2008 (I think it's a Standard but I have no way to know), and a Marshall Origin 20 Combo. I use light gauge strings (.09), a few stompboxes (mostly a reverb and a transparent overdrive) and I'm generally happy with the tone I'm getting.
However, whenever I play any note on the high strings (1st to 3rd) above the 17th fret, the tone loses a lot of texture. It just sounds tinny and weak, with almost no sustain. I would like the tone to be "fatter" and mellower same as when I play on the rest of the fretboard.
I know there are a lot of factors involved (including gear and technique), but I'm hoping perhaps there are some well-known ways to improve tone on those frets.
Edit: I have been using .10's up to last month, and the issues above still happened. I don't think that .10's alone are going to fix my issue.
Edit 2: Been testing this again. I would say I'm OK until fret 17, then it goes tinny beyond that. I have updated the question.