Different tubes burn at different brightness levels. If you have 6 identical output tubes and there is a big difference in brightness then most likly the bias needs set.
The soot on the wall is common in tubes. Don't worry about it. This is unwanted gas in the tubes being burnt up when first firing up a tube.
The soot on the wall is common in tubes. Don't worry about it. This is unwanted gas in the tubes being burnt up when first firing up a tube.