The amp is very, very unlikely to be your problem. You are far better off fixing the offending component. And if it is the speakers causing the brightness problem you can work all you want to use compensating equipment, but the distortion causing the brightness, and the distortion causing the "fixit" components to cover up the brightness, will all just add masking distortion.
In the end, you will end up getting rid of the offending component anyway, you might as well avoid years of trying to fix it and a lot of wasted money, and get it out of there now.
In the end, you will end up getting rid of the offending component anyway, you might as well avoid years of trying to fix it and a lot of wasted money, and get it out of there now.