I found Brent Jessee, a tube dealer close to Chicago in Hoffman Estates who has a great selection on tubes and gives a 30 day full trade in on his sales. I have chosen a matched pair of 12AX7 Telefunken ribbed plate, diamond mark,for $175. They sound very good, even at a very low volume.
This is my first tube swap and I can hear a difference in the cleaness of the highs and the overall improvement in the soundstage.
He suggested possibly trying out some NOS KT88's. I just bought some new Russian Genelex Gold Lions and he said that they are OK, but not in the same league as some of the older KT88 from the 60's. So I may be selling my Genelex on Audiogon pretty soon.
This seems like a way to slowly upgrade to some of the more expensive Amperex tubes that have been suggested.