Questions about retubing BAT VK-75SE

I have retubed the four 6C33C power tubes on my BAT VK-75SE yesterday. Do I need to retube the 6H30 input tubes and the six 6SN7 driver tubes at the same time? Do the input tubes and the driver tubes last longer than the output tubes?
yes, they do. However, you can change the sound slightly with new 6sn7's and the tube next to the 6h30s'. I used RCA 6sn7's and it's softens and sweetens the hights somewhat.

Matching is important on the 6sn7's, as the are part of a balanced gain stage...matched triples for each side.
I also posted on AA and V.K. replied that the input and driver tubes should last longer than the output tubes and don't need to be replaced at the same time. But I just noticed that two of the 6SN7s had developed cracks at the base so I am going to replace them anyway. But finding matched pair is difficult enough; I don't know where I can find matched triples. Do you know know of any good source?
What is it about cracks in the base that you feel make it necessary to replace the tubes? I assume the tubes still have intact pins and hold vacumn. If that is so is there another reason?
I don't know what caused the cracks. I am just afraid it might have been damaged in some ways that I am not aware of. It probably means nothing other than making me feel better.
I ended up not using triples. Went with matched pairs, but use one pair on the left moost 6sn7 spot for the left channel, then the other matching on the left spot for the right channel, then the next matched pair in the 2 middle 6sn7 spots and so on...

I'm with newbee. If the 6sn7's are working...don't worry about the cracks...