[Closed] Rear Derailleur adjustment and setting.

I have been tinkering with a ongoing problem with my rear mech asit seem intermittently reluctant to go into low gear on the rear cassette.
I though I would have a bash at putting new cables in and following the instructions in the magazine pull-outs.
The problem I have is that the rear mech does not seem to want to go across as far as the largest sprocket on the cassette. With the gear cable removed, the high gear stop works fine and can be used to set the top jockey wheel in line with the smallest cog. Letting go of the mech sees it spring across the cassette but wont go any further than mid-way between the largest and next largest cog. The low stop has been fully wound out so it can't be this.
Admittedly the mech is old (original XT from '05) so has the spring gone? I can't see it being the spring as even under thumb/finger pressure I can;t get the mech far enough across.
I did take a photo of the set-uo but cabt see how to paste it here. Hope this description provides enough ifo for someone to help.
Finally, if I need a new mech, woudl it be a stright swop for an SLX or similar that I have seen on CRC. What does "shadow" mean.....profile of functional?
All sugeestions appreciated.