Thank you, max and Andre. You are both correct. The treble control is working. My old ears don't hear the high frequencies.
I did some analyzing: I connected the output of my phone into my DAW and used the Voxengo Span
VST plugin to see the spectrum of the audio. I played back a wav file of pink noise from Poweramp.
Voxengo Span is set to a 3 dB slope so that the pink noise is a straight line on the graph.
The first screen capture is the pink noise playing from Poweramp with the Tone button off.
The second screen capture is pink noise playing from Poweramp with the Tone button turned on and the treble control turned full clock-wise.
Note the slight dip between 4k and 6k Hz. That looks like passband ripple. I can hear the dip when I turn up the treble.
I suggest changing the Q of the filter to eliminate the passband ripple.
Another suggestion would be to have a switchable center frequency, say 10K, 8K, 6K for us with old ears.
Poweramp is the best. The graphic EQ is perfect and sounds very good.
I will use the graphic EQ to adjust my treble preferences.