KT88 and 6550 are pin compatible. EL 34 can only be used if the sockets are wired for triode mode operation(grid 3 to cathode, pin 1 not connected as with KT88/6550).
I use the EL 34 for driving a Lowther in an OB (400-20kHz). The same amp sounds best with the 6550 connected to front loaded horns (400-4.5kHz).
The KT 88 sounds more silky in the upper mids with a wider image, the EL 34 has definitely some coloration but sounds very warm with wonderful sound colors, the 6550 sounds very dynamic and clean with more punch. Very nice for driving an active KH bass.
All 3 tubes have a different plate resistance so the sound in the Citations might also change because of this but you could influence it by connecting the speakers to a lower or higher output impedance (if the speakers are easy to drive ) which will change the impedance of the OPT primary as well.