Sony str db1080-loud noise on speaker A

Dear Forum memebers
Apologies if this question is a bit low brow and easyto answer. Just it appears hard for me to fix.

I have a Sony str db1080. I have ben using it soley with a pair of Paradigm speakers mainy for listening to music.
When I first set it up I had left the channel selector to A+B. And thought nothing of it as I was only using 2 channel Stereo.
I have finally gone out and bought a centre speaker so that I can listen to movies a bit better.

However, when I put the AV on A+B it comes through only as 2 channel (as the manual says-so OK). However, when I out it on channel A only 1 of two things happen.
1. 2CH-Sound is OK but no centre speaker
2. Auto detect-dolby digital light comes on, LFE etc, centre speaker is recognised (on front of AV led doisplay) but there is a god awful buzzing humming noise.

I have discnnected, reconnect. Gon trugh the testtones (and the centre speker works OK).

I really am at a loss on this one?

Any help would be appreciated. I hope it is something really obvious (though embarrising for me) to fix.

Regards

David

greetings

The noise comes from the speakers themselves. You can faintly hear the sound of the what is being played but the noise the the speakers is absolutely drowning it out.

Regards

David