kindasonic, the SRGII is IMO a very nice sounding amp. Did you get the PSU1 with it or the switching PSU? If the second make sure the SRGII is grounded using the grounding post on the back of the amp. Good luck.
I have the HE300 and IMO is a great sounding can, better than the Senn HD580/600. I see charts and they are tempered by what I hear, I go with my ears over the charts but that is just me. BTW, I do not use the stock cable instead I use the OCC HE6 cable terminated in balance 4Pin configuration.
As far as the K550 I cannot comment since I have not heard them.