The Arcam is a nice unit. I recently auditioned the 300 for a friend who wanted a system for his boat (long story). As you noted already, it is about the same price as a 990+amp. For the money, I think the 990+amp is a better deal as I feel the 990 is a little more transparent. Also, the M5 is quite inefficient...probably quite taxing on the amplifier stage of the Arcam at anything close to reference sound levels. The AVR300 is only rated at 100 watts/channel @ 8ohms when all 7 channels are driven and only goes to 120wpc if you're only driving the mains.
Good luck!
_________________________
.signature