Overall the Sig is not that finicky. I am surprised you are not getting good results.
Perhaps you need to give your ears more time to adjust. The 600 and 650 are notoriously dark and require some decent amping to really shine, the Sig on the other hand is very much at home on a ipod or a megabuck uber system.
Bottom line.....give them a chance.
I didn't mean "not getting good results". It is quite good but probably not a wide enough soundstage and a touch to bright. That might be better with a different amp/dac combo.
It is probably wrong to compare the closed SP with the open HD650. Both are good just different !