Single Ended Amplifier 6550 KT88 UL Triode Is Good but EL156 Is King
6550 KT88 (triode or UL) Single Ended is good, but EL156 is king. In a single ended class-A an amplifier is simply to enjoy all the music, if you have a good speaker system and C/P and 300B SE valve 300B is a low cost. But speakers need at least 91dB/m/w.and range speakers like B&W, Klipsch, etc.
KT88 and 6550 are almost the same charactor so that you can use the other the same amplifier.
but a tendency adjust KT88 not always sound the same when Maker is different, please try.
Each way through Penthode less Triode you please try to use any type of driver.
The SRPP as a triode 12AX7 (ECC83) 6DJ8, one of the race with the EF86 penthode, 6SH7 not.
3.5K ohms when using high voltage to drive the resistance voltages.cathode
about 390 ohms 10W ~ 20W. Just try each driver type by UL and triode wired ride.