I have tried 6550's, KT88's, el34's, kt77's, and recently the KT120's in the Cary. They all have their merits, but with the right speakers I prefer the EH el 34 the most. I think in triode it has a wonderful balance.
However the lower wattage of the EL34 just could not drive my latest speakers properly. The KT120 tubes went a long way to fill out the bottom end and overall presentation. I know they do not increase the power output, but they definitely present a much fuller sound with my harder to drive speakers.
So it all depends on what you are looking for, and what speakers you are trying to drive.