The Sovtek and Svetlana of that tube were both made in the the same factory. You will not notice a difference.
You should use the EL84M but some other EL84's will work as it is a heavy duty version of that tube and they recommend it. The Phillips SQ is a very good tube for this and will make things more musical over the standard Sovtek.
I recommend changing the 6922 tubes to Siemens or Telefunken.
You will need to rebias the amp after changing any of the tubes. The El84's should be biased first at 20 volts. If you are going to change the 6922 make sure you put them in before you bias the EL84's. The Power tubes should be biased at 700 mv. Higher bias will make it softer (classic tube sound) lower than 700 will give you more dynamics and punch.
You should use the EL84M but some other EL84's will work as it is a heavy duty version of that tube and they recommend it. The Phillips SQ is a very good tube for this and will make things more musical over the standard Sovtek.
I recommend changing the 6922 tubes to Siemens or Telefunken.
You will need to rebias the amp after changing any of the tubes. The El84's should be biased first at 20 volts. If you are going to change the 6922 make sure you put them in before you bias the EL84's. The Power tubes should be biased at 700 mv. Higher bias will make it softer (classic tube sound) lower than 700 will give you more dynamics and punch.