Hi, I received my 650 yesterday and overall I am thrilled with it, I think it's GREAT! However something is bothering me, the two buttons underneath the directional buttons, the Mute and Return buttons, these are VERY unresponsive often requiring multiple presses or very firm pressure to achieve a response. Please may I ask, has anybody else found this? as I have asked for a replacement form Amazon, however, I feel there is a GOOD chance I am wasting my time, and this is the same on them all. Any information or ADVICE would be very much appreciated MANY thanks.