I've had good service from tubedepot.com. They ship fast and answer emails quickly. Great if you already know what you want. Sovtek is not known for refinement, better for reliability (think guitar amps on the road.) I would choose the EH between the two. Still not the ultimate in refinement, but good for the money and reliable.
Jim McShane has some vintage 6H30 listed on his website, he'll know the skinny on the current production tubes.
http://pages.prodigy.net/jimmcshane/tubes.htmIt always pays to ask Bill O'Connell too. His collection never ceases to amaze.
Since you only need one tube, and used in a source component, I would try to get the good stuff. Bigfish is your man; he has been swapping 6H30s recently, I think he settled on DR. Whatever brand Audio Research uses in all their gear sounds fine too.