If you love the sound of your HD650, chances are you won't like the HD800. Some people on K701/2's gave feedback that the AKGs are more "musical" than the HD800, although I suppose that depends on the amp and DAC/CD/TT too.
I have the HD600 and got to compare with the HD800, both on an X-Can V8 and a Corda Cantate2. The Cantate is a little drier than the V8, but you'd strain to hear it on most recordnings, on the HD600. On either amp the HD800 is noticeably drier than the HD600, BUT as far as I'm concerned, while I liked the HD800 for some recordings, the difference isn't big enough for me to warrant the added expense. So while generally changing headphones/IEMs/speakers would do better to change the sound signature, if you already like what you're using now, then maybe you should look at the other equipment in the system too.
In any case, what's with your HD650's reliability record? I have a five year old HD600 and it's never been broken, not even the stock cable.