Taming a Bright Phono Section

1. Cartridge loading, as others have mentioned.

2. Use the tone controls, as others have mentioned. (They are specificly made for the purpose of fixing your problem).Be sure that the knobs are installed correctly. You might have the treble boosted without knowing it.

Did you get the very comprehensive instruction booklet that explains how to adjust for best sound? If you need it, I can search for mine, and make a copy for you.
Never bright in my experience, with Shure, and Grado MM cartridges.
Is there any trouble with brightness listening to CD's through the linestage?
I have two non-bright, proper sounding units in storage, as I could not bear to part with a "classic" design, when I converted to tubes. Another is still being used by a family member.
The suggestions above seem to cover most possibilities.

The sound through the line stage is NOT bright. Plus I have set the
correct cartridge loading per Ortofon specs. I'm beginning to think
someone might have doctored the phono section. Or maybe it's time to upgrade the passive parts in the PS.

Good cones like the Audiopoints might help a lot when used in combination with internal cabinet damping. You could be surprised! You could also be listening to electrical noise. Make sure your power is clean before drawing any conclusions.