Your 555's will benefit, but I am not sure you will notice it without having previously spent time listening to them on a lesser source/amp.
However, the STX/e7/e9 wont drive HD650s as well as you think. The gain will be there, but the drivers wont really come alive in the right way.
Best way i can phrase it is that the bass and the volume will be okay, but your going to notice some dullness or flatness in terms of sound if you compared them to a pair running on a better amp. Its similar with my AKG 701s which I have spent alot of time with on the STX, my yamaha receiver, and my matrix m-stage. They don't have an insane impedance like 600ohm beyers, but they do love current.
There are better options out there than going straight to 650s if your looking for better SQ in a relatively easy to drive package.
On the flip side of it, owning HD600s/650s with just an STX and then upgrading to a external amp running off the STX's preouts will be an incredible audio experience. The STX is a solid source, unless you plan on spending $300-350+ for a DAC.
Im personally just using the toslink out off mine at this point, which is a complete waste of the card since I could just get it off the onboard, but I suppose there is no point in letting it go to waste, plus it looks sick in the case Edited by rmp459 - 5/4/11 at 7:26am