It sound as if your overscanning on the red, the red is being clipped by the edge of the tube...
[WARNING] - the following is dangerous.... to your vision...
Turn the contrast down so that the image is just visible when you look into the tube and see if the image is being clipped by the edge of the tube.
[/WARNING]
(Keystone correction can cause clipping in a corner)
If you have a burnt tube, it is usually burnt in the middle, so on an all red image, the center will be darker than the edges - the only exception is if there was a permanant logo shown, but this should have been just been noticable when you looked at the tubes when buying
I would sugest that you follow all the instruction on setting the unit up and correcting the convergence with a standard sized image. and checking this for 'burning' before increasing the size of the scanning..
For a bigger image move the projector back...
Sometime if the PJ is at a large angle - the image size is reduced...