"Auto" mode on Marantz amp stopped working

The "Auto" mode on my Marantz SR5200 amp no longer provides useful output. i have a DVD player connected to the amp and when I used to select "Auto" I would usually get 5.1 output (if the disk was supplying that data). Using the same source material, the "Auto" mode now only uses the front two side speakers (ignoring the centre) and plays at a very low volume.

Now when auto mode is selected, the amp's display shows "PCM", "Stereo" and a red speaker symbol below where the speaker symbols are shown on the display. This symbol can be seen here. The image shows the same amp set to tuner (which mine isn't) and with the red "Stereo" and speaker icon which I can't get rid of.

The change was brought about by a delightfully enthusiastic button-pressing session by my 5yr old and the remote control.

Anyone able to assist? I've been Googling the problem for longer than is healthy

Turns out the button pressing was purely coincidental and thus a red herring. Sorry.

One of the two front speakers wasn't fully connected. I've reseated the connections and it's now fine. The other speaker (a KEF HTS 2001) seems to have completely died. I can only get a very faint sound out of it. When I switch the speaker with one of the others I get exactly the same response so I'm sure it's the speaker.

Unsurprisingly when the amp's "Auto" mode was switching to stereo and trying to pump sound through one poorly connected speaker and another dead one, we weren't getting a wonderful audio experience

Now I need to replace or repair the dead speaker. I can buy a new one in the US for $130 (here) but I'm not certain whether I should beat myself up trying to find a Kef "Egg" or just get something entirely new and use it as a new center speaker. Thoughts?