Here is an experiment you can try for free if you can play your headphones from your computer.
Download the DC6 demo, it is fully functional but limited to one minute of recording or playback.
http://www.diamondcut.com/Downloads/Downloads.htm
When you have the demo installed open three instances of DC6.
In the first instance go to Edit, Make Waves. When the make waves window opens make sure that the Linear Sweep box is unchecked and the Sine Wave button is selected. Then put in for frequency a low bass tone, something like 53 Hz or so, set level to -20 db and length to 59 seconds and then create the wave file.
In the second instance of DC6 do the same thing but set the frequency to a high treble, say six kHz with the rest of the settings the same.
Play the treble wav file in the second instance of DC6 over your headphones at a loud but not uncomfortable volume. You may have to go to Edit, Preferences, Sound Card to select your sound card.
Cut and paste the bass wav file created in the first instance of DC6 into the third instance of DC6.
Then cut the treble wav file from the second instance of DC6 and mix paste it into the bass wav file already in the third instance of DC6.
Play the mixed file back over your headphones at a loud but not uncomfortable volume while listening for differences in the treble note.
Create louder and louder bass wav files in the first instance of DC6 and continue to mix paste them into the treble file from the second instance and play the mix in the third. Three db steps should be about right for the increase in volume.
Does the treble note change character with increasing volume of the bass?