Why does my guitar sound horrible? ?

I've got a cheap f310 and a very expensive takamine. Intill now I havnt restringed my takamine but I put some 12,s on ( prosper bronze) and it now sounds really cheap and tinny. And I used the exact sane ones on my f310 and it sounds brillant, well a hell of alot better than my nearlly thousand pound takamine. Please help me I'm really ticked off !