I own the SP200+M200 stack and I can totally recommend it (if you like the slanted design). It works well with everything I throw at it, DT880 600Ohm, HD650, Sundara (which you mention), no problem. For fun, you can use it to drive Sundaras from the balanced output, completely unnecessary, but so is most of what we discuss here
. If you get a unit with the feared channel imbalance issues, you can always return it.
I think most of the options we are talking about here are absolutely ok, which is awesome. Someone mentioned the RME ADI-2 DAC and yes, I can confirm that that is an option that gives you significantly "more", and it's no coincidence it's priced above your current 500EUR limit. Within that limit, there is plenty to choose from, imho.