I have tried Normalising it down by -2.5db and also by equalising it FLAT by -3.5 db.
With the equalizer set to "flat", you are NOT equalizing.... You are just reducing the volume.
Normalizing is also just a volume change, no different than reducing the volume at playback time... It doesn't change the "character" of the sound.*
After normalizing ,( and I need to do it to -7db), , if I go and do amplify ( without clipping) I am really back to the same levels as before i normalized the file.
With the
Amplify effect,
you can choose the amount of amplification (up or down). By default, it will be set to whatever gain is needed (if any) to bring your peaks to 0dB. You've reduced the volume by about -6dB with Normalization and "Equalization", so it's not surprising that Amplify defaults to +6 or +7dB to bring the volume back-up.
I have a few tracks of old music in MP3 format. I find it very noisy and shrill.
As Gale says,
bass & treble adjustments or equalization are probably your best bet. The
Graphic EQ mode is generally easier to experiment with than the
Draw Curves mode. This is just a
wild guess, but try pulling the sliders
down between about 1kHz and 4kHz. Start with about -6dB and experiment from there. That should remove some of the shrillness.
I have a few tracks of old music in MP3 format. I find it very noisy and shrill.
You may have low-quality low-bitrate MP3s. If that's the case, you may be able to make some improvement but
MP3 compression artifacts are impossible to remove. 
i.e. Violins encoded to low-bitrate MP3 (or cell phone compression) are never going to sound "natural". And who knows... This may have been a poor performance on a cheap violin or a poor quality recording to begin with...
What is the file size and playing time of one of the MP3s? From this, we can calculate the bitrate (kbps = kilobits per second). There are 8 bits in a byte, so:
Bitrate in kbps = 8 x (File size in kB / Playing time in Seconds)
This calculation is approximate because the file header takes up some space and there may be embedded metadata & graphics also taking up space.
Good quality MP3s are usually around 200kbps or more. I think MP3s downloaded from Amazon are 256kbps, but they may use variable bitrate. The maximum MP3 bitrate is 320kbps. 128kbps
may be acceptable for some music. (You can go lower for voice, or if you are not concerned with quality.)
* There are cases where the peaks can go above 0dB. But, "normal" WAV files and and your digital-to-analog converter are limited to 0dB maximum. In that case, normalizing (reducing the peaks to 0dB or below) will prevent clipping (distorted flat-topped waves).