Kris, tone controls can vary both in the amount of maximum boost or cut provided and the frequency at which that occurs. Most receivers have that frequency at a set number(one of my receivers has a choice of three frequencies), usually around 10KHz for maximum treble boost/cut. What you describe would result in the cut beginning above about 1KHz, sloping down about 3dB at 5KHZ, reaching 5dB down around 10KHz and then staying about constant above that frequency. If the problem is upper mids/treble, there's no reason to also cut the bass/lower mids.
Yes, the tone controls affect all inputs when they're adjusted; if a different setting is wanted for a different type of material it has to be changed manually.
If Randy's suggestion of further room treatment isn't feasible, as you seem to indicate, then use the tone controls to get the most pleasing result; tone controls are there to be used when necessary.
