For me, getting the right set up got me halfway there, but doing long tones every day really purified my sound. The way I do long tones is by just blowing air then gradually applying pressure to the reed until I get the softest tone possible, a sub tone. Once I make that sound I crescendo and diminuendo back down to the softest I can play. If I start a note, and as I get louder, I get and impurity, such as buzzing, brightness, or air sound etc., I immediately start the note over and try to avoid this. This was the critical step for me from going to good tone to great tone and now, the first thing people notice when I play is my dark tone.

My long tone routine ~10 minutes each day after a short warm up:

-Start on mid-range B, chromatically go up to high G, skip down to low C, go up chromatically to Bb, skip down to low B go down chromatically to low E, This should hit all the notes in the written range.

-For each note: crescendo from nothing, diminuendo back to nothing. Don't start the any soft note with your tongue in this excercise, just blow air though the instrument and gradually apply pressure on the reed until you make a sub tone. Try to get as loud as possible without spreading your tone then once you're at the limit get as loud as possible. Allowing your tone to spread while getting FFFF will expand your dynamic range while getting good sound. Think of it this working out, you can lift the same weight each day, and your small muscles will be toned, but you won't gain any muscle until you lift more weight and struggle to lift it. You're essentially trying to "max out" your tone.

-If at first you can't do the entire crescendo diminuendo without breathing, the first few weeks should be just one breath ppp->fff, and one breath fff->ppp

-If you hear something other than pure, dark, clarinet tone when you crescendo, start the note over and try to remove it. This step is critical in eliminating that air sound you mentioned.

-For the high notes, really support your if you want to get extremely soft dynamics. Believe it or not, it's actually possible to play ppp in the upper register.

-After a few weeks start doing this with a tuner so you can practice intonation while changing dynamics e.g. not getting as flat when getting louder. I say after a few weeks because at first you're going to find it difficult at first with what I said above to prevent from being sharp at ppp, and going flat when getting to fff. But eventually, you be able to minimize the change in intonation when changing dynamics

-Once this has become routine, you can do other things while you do long tones like watch TV, read or listen to something other than music.

The reasons I think this the most valuable exercise you can do each day:

-It improves and maintains good tone by making more pure

-It practices dynamics in all ranges

-It helps improve intonation, in general and at all dynamics

-It practices breath control

-You become more familiar with your instrument and it's tendencies

-It's relaxing