Apologies, the room is is 2.7m x 5m and because there is a door in the centre of each long wall the system has to fire down the length of the room.
It is a brick built extension on the side of the property so with the door into the property shut I can play music at pretty decent volumes without disturbing the rest of the house.
The LS50Ws are an amazing system and they work really well in there and from what you guys have said I should be able to set up exactly what I was hoping for. First steps are to sell the current system and get the LS50Ws installed. Once that's done I'll be asking for specific advice re an AV amp. Many thanks for all the help.
You should be good to go then, and can expect superb sound quality from those speakers.
Thanks to William for answering some questions for me yesterday. Some of us would typically give guarded responses such as by cautioning about the room size, sitting distance, required sound pressure level (how loud) etc., especially when the speakers in question are near field monitors such as the LS50/LS50W.
If you don't sit too far from the speakers, say less than 10 ft, you could actually approach "reference level" but it sounds like you don't listen that loud anyway, so again, you will be fine. Below is a link to an online peak spl calculator you can play with, and a screen shot that shows the spl you can expect from the LS50, sitting 10 ft them them.
You can use that calculator to size the AVR, or prepro/power amp you need for the remaining 3 speakers.
http://myhometheater.homestead.com/splcalculator.html