Yes, Lundahls though K&K are a great deal (plus very small fee an assembled box); in my system I prefer the Sky to the copper LL1931, but I would absolutely love to hear the silver-wired LL1931Ag or LL1933Ag (or their higher gain versions) sometime. The LL1931 is certainly a great value for the money. Very transparent, airy, and detailed; a refined and grainless sound with an awesomely huge soundstage. The Skys give up a little on those things, but in return supply more body, warmth, and bass impact. I wonder how much of that sonic difference is attributed to the amorphous core (i.e. would the laminated metal core LL1933 sound closer to the Sky?). In my opinion the Sky makes the Koetsu sound even more "Koetsu", but it’s valid that some would like to take it in the other direction. The Skys are also slightly smoother and more refined than the older 1131 CineMag.
I agree that acquiring a great SUT is a good investment that will never wear out.