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Congratulations on getting a Jadis Orchestra, via the fact that so few people come across them, and Jadis products in general, it's one of the most underrated products in high end audio.
Remove the bottom and both side plates of the amp. For convenience sake, you can put it back in a way that allows the bottom plate to simply come off by itself, which really simplifies the procedure. I wholeheartedly suggest you do so when you reassemble the amp.
There are 5.62 Ohm resistors between the plate of each output tube and the output transformer. The more readily apparent way to identify this resistor is that it's connected to the yellow wire coming off the output tube. Measure the voltage across this resistor, and adjust the tube's potentiometer to a range of 90 - 120 mV, with 110 mV being the ideal value specified by Jadis themselves.
There is also a more esoteric technique in addition the adjustment in the preceding paragraph, where one tries to balance the two tubes in each channel in the way Fender switched to with their guitar amplifiers in the mid-1960s. It's a bit more complex to explain, and many feel (including my friend at Jadis) it unnecessary, but if you really the burning need to understand it, let me know.
Hopefully, Velo62 will provide the photos to you. As they say, a picture is worth a thousand words. Otherwise, I probably can get something to you.
Enjoy the Jadis - I wish you the very best of luck with it!
I've not encountered an Orchestra Signature yet, but based on it coming with the KT90, you have the transformers that allow you to use any of the tubes on your list. You cannot use the 6L6 (according to Jadis) as there is too much plate voltage (my statement, not Jadis') for that tube to handle.
My personal experience/tastes are that the EL34/6CA7/KT77 are the best sounding tubes for the JOR. And, I like the magical midrange of the JJ EL34/E34L better than their more bottom heavy KT77. I'm not much of a fan of the Ei KT90 Jadis uses or more or less any 6550. I've not tried the EHKT90, though they certainly have their legion of fan ala some of the more recent Rogue designs, which I do greatly respect. Of course, this is all a matter of personal taste.
Finally, one thing I neglected to mention above is to NEVER operate a tube amplifier with output transformers without a load, as you can blow the one part that is proprietary (in addition to likely the most expensive) to just about every individual product.