Will changing gain affect frequency response?

Hello again, Erik.  Thanks for your time considering this.  I'm not arguing the F/M curves either....more a case of not sure they explain what I think I hear when gain is increased but volume reset to the original level.  

Ghosthouse, yes, you're right that the Fletcher-Munson Effect will not cause a change in perceived tonal balance if the volume at the listener's ears remains constant.

Regarding your original question, yes it does seem conceivable that changing how the overall system gain is distributed among the various circuit stages that are in the signal path could affect perceived brightness.  Although without knowing the details of each of the designs that are involved a more specific explanation would be speculative at best. 

For example, Atmasphere has pointed out in a number of past threads that very small amounts of certain harmonic distortion components (e.g. the 5th, 7th, and 9th harmonics) can affect perceived brightness.  Changing the signal levels that are processed by the various gain stages that are involved could conceivably affect the degree to which those distortion components are generated.  Erik mentioned another example, which is that impedances might change, depending on the specific designs.  If the output impedance of a component were to change significantly, and is relatively high, its interaction with cable capacitance could affect treble response to a perceivable degree.  And it seems conceivable that changes in impedance at internal circuit points could also affect the treble, depending on the specific designs. 

Best regards,
-- Al
 

Thanks, Al.  Does sound like there are few possible hardware-based explanations for what I thought I heard.  

Seems I missed(spaced) the part where you brought the volume back down, at the listening position.    Kinda obviates Fletcher-Munson from the conversation(sorry).

It's like a glass of water, if you fill up all the way, you will have to limit yourself on the speed of carrying that glass of water because not enough 
'headroom' left to keep water stable inside.
If you do not fill up, than you might be still thirsty and desire more water in your glass. So the goal is to reach sufficient level of water so it doesn't spill and it fills you up.