chris hollands:
Is it worth an extra £400 for a Monitor Audio RX6 av12 over the BX5 av ??
Yes, most definitely.
chris hollands:
And if so would it be o.k with a Yamaha RXV667.
Yes, although you really need to be looking at the £1k range of av receivers as a minimum if you want to hear the best from the RX6 AV12. Think Onkyo TX-NR808 at least.