We went back and forth on a couple of our vintage Marshalls with the KS. For me EL34s are THE sound. They saturate and are voiced like what my vision of a great Marshall is. I think the 6550s are more raw and raunchy. The difference was in my amps the 6550 reminded me of The Who, JET ect.. and the EL34s reminded me of Eddie, AIC, Hetfield, Yngwie. YMMV