can't tell ya about 6550 sovts but EH to my experience has a record short life that may be very distructive to the component circuitry.
i've used eh EL34 and will never place them again in my VTLs. i now use either svets or yugos. during 1+year period measuring the bias i found that it didn't change even slightly.
therefore i suggest
#1 ask manufacturer if it's OK to go with EH since some of the brands are OK... i assume that EH lack the lifespan to all other tubes significantly in any component
#2 find yourself the best match for performance/reliability and nevertheless budget as well.
i've used eh EL34 and will never place them again in my VTLs. i now use either svets or yugos. during 1+year period measuring the bias i found that it didn't change even slightly.
therefore i suggest
#1 ask manufacturer if it's OK to go with EH since some of the brands are OK... i assume that EH lack the lifespan to all other tubes significantly in any component
#2 find yourself the best match for performance/reliability and nevertheless budget as well.