I did design the 20W amp that was used in the mini-Jubilee, the original Origins and some other amps that did not see the light. That design DOES work with any octal tube, even 6V6. I don't know what circuit is in the Studio series but I'd be very surprised if it isn't just the same as the mini-Jubilee with the preamp modified accordingly. In fact those amps were already planned when I was still in Marshall and that's what was being done, just a preamp change (which is actually the same if you think about the 100W amps, same power amp, different pre). Regardless of circuit changes, the amps will work with the other tubes. Actually the EL34 is harder on the transformers than tetrodes like the 6L6, 5881 and so on. The KT88 and 6550 draw a bit more of heater current but that was taken into account when specifying the power transformer. In Marshall's defense it is normal that if the amps come with EL34 they told you to only use those but that doesn't mean it won't work or will be dangerous to use other tubes. I'm sure if you ask if you can use a 12AT7 in the preamp they'll tell you it is designed for 12AX7 too. Hmmm, a KT66 should work in the SL-5, perhaps it runs a bit cold and it is not optimal but definitely should work.