Tube handling/designations

You do not need to use all the same brand and there are certain positions that will affect the sound more than others. The 7308s and also CCas are generally, but not universally, considered to be the best of the the 6DJ8/6922/E88CC/ECC88 family of tubes. Google Joe's tube lore to get info on the sonic differences btwn diff brands and members of the family. And also be aware that BAT products are not GENERALLY considered to be as susceptible to dramatic changes due to tube rolling. Also be careful of counterfeit and dramatically over-rated (meaning sold as NOS but barely functional) tubes on (Fl)ebay!! You should consider a tube tester and get return rights on tubes. A legit seller will provide that. Also talk to members Kalidekol and Jsautter for tube sales and good advice.

I would recommend the Siemen CCA's in your VK 30. They are among the best in the 6 volt 9 pin family. I have not had a good experience using the 6dj8's in the BAT preamps. Other peoples experience may vary.

Tubes will not burst the way a halide bulb will burst from skin oil depositing on the tube. With halide bulbs, the oils are subjected to extreme heat which causes the oil to break down and carbonize. The black deposit that results will then absorb much more of the light being put out by the bulb than the rest of the glass on the bulb. The difference in temperature between the other part of the glass and the part with the black spot causes stress in the glass which can lead to failure. These bulbs are under extreme stress from the high pressure gas in them.

Tubes do not operate under similar conditions -- heat is lower, no significant light output to be absorbed by a spot on the glass, a vacuum instead of high pressure, etc.

Still, I don't see why caution would hurt, particularly with hotter tubes, like a 211 or 845.

Heat is the enemy of tubes- they ought to be kept clean of dust and fingerprints for the longest life. 6DJ8s and 6922s are capable of running a fair amount of current and so can get quite hot.