I have had my Onkyo 508 for a couple of years now and have never been truly happy with the amp so i'm thinking about upgrading it. I have been offered a brand new 616 for £449.99 but would like a 609, which I can get for £349.99. Or there is the 515 which I could get for around £400. Would there be any noticeable difference with these amps over my current 508? Though I would like to upgrade I really don't want to have to part with my money if I there is not going to be much of a difference. Any advice would be very much appreciated.