Picture dims when Theatre Sound activated

[pabutle30]

I have a KDL-52HX903 connected to a BDV-E870 and a Sky HD box and Playstation 3, when watching TV and wanting to hear the sound from the surround system I press theatre mode but the picture dims significantly, and I think a message pops up saying picture mode changed.

How can I stop this dimming?

Also I seem unable to set the enhanced bass function as I find the sound slightly tinny

Thanks