Linearity and clarity for the most part. These are the SED6L6GC which have a good rep and are very rugged. (Caveat, there are lots of 6L6 types so if you are investigating them, be mindful of the differences, especially in the Baron. FWIW my Baron is biased for the old Sovtek 5881's so I have not heard them in it, but I have heard the 5881's and the 6L6's in my Primaluna and there is no comparison. The 6L6's are much, much, better.) They are neutral and very natural in this amp.
Hope that helps you a bit.
Hope that helps you a bit.