how much tubes influence on the sound ???

hi friends .
im new in the tubes world all i know that there is alot of kinds of tubes .i have cary sli80 amp with devore nines speakers ,i bought my amp in seconed hand and i dont know when the tubes was replaced or .. , all i know i have this tubes in my cary sli80 :
2 6sn7 elctron tube made in china.
2 6922 sovtek made in russia.
4 output tube svetlana electron devices 6550 c made in russia.
2 5u4gbeh electro harmonix masde in russia.

i see on all the tubes in the top alittle black color on the tubes i remember this since i bought this cary sli80,
and sound is ok ,why is that color on my tubes or how can i know when to change them ?
another importent thing is in my topic question is the tubes really change the sound that everyone can feel the change even people that not audiophiles ? i ask that bcz my dealer sugeest me 2 tubes 6922 of EAT that coast toghter 350 $ and if i buy the others tubes that less importent to the sound then the pre tubes is 250$ for all others,is it worth this my cary sli 80 that is very averge amp for this lovely speakers (devore fidelity nines).

there is nice friend here in the forum that suggest me this tubes to my cary sli80:
2 Amperex 7308 input tubes pre ----> is this better then -->eat tubes 6922 ?
2 sylvania 1952 6sn7gt "badboys" pre driver
4 goldlion kt88 output tubes
2ge544 rectifier

how much this set of tubes will coast me ? and is my amp worth this investment (honestly pls) ???
thanks alot and sorry about my long questions i will be glad to hear alot response to my all questions and by the way i love this audiogon forum very nice people and proffesionals people here bye and good day :-}