My HD-650's soundstage definitely got larger when I upgraded my amp, but I wish it had the huge soundstage of the HD-598. If someone says the HD-650 can have the same size of soundstage as the HD-598 then I am going to say you are lying
I find it hard to believe that Sennheiser intended the HD-650 to have a huge soundstage anyway. No problem there.
For me the largest soundstage I've heard is the AD700, then the K702 and HD-598. People always said the ATH-AD2000 had a small soundstage but I felt it was larger than that of the HD-600 and HD-650!
Personally I don't need a huge soundstage, but when it has the mids of the HD-598, it's kind of addicting.
Depends on your source amp etc .. I can plug in my HD650's and have a huge sound stage because my DAC puts out a huge sound stage each one of your components affects the final outcome