So much of the SQ depends on the amp the tube is used in that any comments have to be very generic and unspecific. That said,
I have found the EL34 usually has a warmer mid-range than the other 2, the KT88 has a warmer upper bass/midrange and a tad more sparkle in the highs, and the 6550 is the more neutral of either providing a uniform response through out the range.
Much of which tube/amp combination you prefer will have to do with your speaker/source selections and the brand of the tube, they vary substantially within each type. As you will see, others with different amps than mine will have varing opinions. There is no pecking order - each tube will have its own adherents.
I have found the EL34 usually has a warmer mid-range than the other 2, the KT88 has a warmer upper bass/midrange and a tad more sparkle in the highs, and the 6550 is the more neutral of either providing a uniform response through out the range.
Much of which tube/amp combination you prefer will have to do with your speaker/source selections and the brand of the tube, they vary substantially within each type. As you will see, others with different amps than mine will have varing opinions. There is no pecking order - each tube will have its own adherents.