I bought an EL-34 based Jolida 302BRC integrated tube amp around two and a half years ago. Last year I called Jolida to discuss going up the food chain. The rep said I could add another 10W output power by simply changing the power tubes on my 302. He said my 302 has the same guts as the 502. The only difference was the biasing LEDs were set for 400mv biasing. If I converted the power tubes to KT-88s I would have to manually bias the tubes to 500mv. He said I could even use KT-120s or the new KT-150s - although I would not get their full benefits since my amp was not designed to handle their full power. I installed some Genalex Gold Lion KT-88s and biased them to 500mv. Wow! Much more dynamics, bass and high end clarity. I usually keep the volume to around 75-80db. At that level the KT-88's just seem to have better control of the speakers and more dynamics. I am going to change back to the EL-34's in the near future to compare how they handle acoustic music and vocals compared to the KT-88s. The EL-34s should make that kind of music sound better. If so then I can swap tubes to best suit the music of the day. I do not know if all 302s have 502 guts. I recommend you contact Jolida before trying the KT-88s.