So, I wasn't happy day 1 with the stock tubes in my DT50. 

Initially, on the preamp side, I went with JJ ECC803s / High Performance 12AX7 Gold Pin (matched and balanced).  While they were pretty good, I felt that they were a bit on the harsh side.  Defintely upped the output, but it wasn't always as smooth as I'd have liked.

After reading several reviews, I decided to give a fairly expensive 12AX7 set a try - TheTubeStore has a set of tubes from Northern Electric that I'm now running.  I think they have a much nicer sound to them.  I have since ordered a backup pair.

On power tubes - initially I went with Black Sable SED 34's (matched & balanced).  The sound from these is quite good.  I found that they held up well to gigging and regular movement of my amp.  For kicks, I recently ordered a set of Tung-Sol EL34B tubes (matched & balanced).  I've only had about 10 hrs on them, but I believe that they sound slightly better than the SED EL34's, and they're about 1/2 the price.  I think they have a slightly better breakup and a bit more harmonics that are very pleasant.

Just throwing this out there for people that wonder about trying different tubes.

Jay