If you are looking for a fuller sound skip the 7119 (5687 replacement) suggestion @ AA as this will crisp things up a bit. Mullard, Amperex White label or Philips Miniwatt 6922's are the ticket for full mids (Amperex Bugle Boy and Orange Globe 6dj8's would be my second string choice). The input tube (6922 type) will have about as much impact on the sound as the output tubes. Siemens, Telefunken and Tungstram 6922's are not a good choice for your stated goal, though they are still good tubes for other applications. The 5687 has less of an impact on the sound but Sylvania Gold Brand 5687's are one of the smoothest/fullest with RCA Black plate 5687's being a bit rougher though still quite fat sounding. Tung Sol 5687's (most versions) are great in other amps, but not the Audion as it does not require their added detail. The TS's have a tendancy to sound thin when used as a driver in Audion's. Maybe just start with the 6922 type tube and run the stock EL34's while you research the output tubes. The best are not cheap, which would be NOS Mullard EL34's (the 1 or 2 version). You might look into EL34's in general (again for your stated "lush" midrange goal). The KT88's will beef up dynamics if you feel that this area is also lacking on certain types of music. Can't remember if it runs KT90's or not, but if it does look into these as well (might be sweeter than 88's, but this depends on the tube).
