Depends how old the tubes are. If too old you should replace or get them tested.
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I have tried KT90, KT88, 6550 and 6CA7 on my Jadis JA30 mono amps (should be pretty similar to your amp) and honestly, there is not a huge difference between the tubes in my amp, even though they are said to be very different from each other. The smaller tubes doesn't normally need to be replaced very often, every four or five years or something, maybe more, depending on how much you use the amp of course. Four KT88's shouldn't be too expensive, like $250 (Genalex Gold Lion).
What tubes do you have at the moment?
After the responses I did not do much with it since the likely hood of improvement seems limited and unclear to me at this point.
Instead I overhauled a HH Scott 299 with new caps restored to all original and some new tubes driving Klipsch-cornerhorns and that set up sound balanced and as good as I would expect from that combo. (the mid-range on that is nowhere near the Jadis DA50 set up but it plays very musical).
Still running the KT90's on the DA50 / legacy Illusion (one was blowing up but had an extra one and got it running). ( they tested Ok ). Four KT88's (Genalex Gold Lion) may be the next attempt.
I am still not happy with the performance for my taste. I head a great sound experience with Jadis amps during a Demo long ago and my set up is just falling short.
The mid-range is good a touch to antithetical (would like a bit more musical) but just a touch off. Some users push the tubes with a higher Bios or so not sure if i want to go in that direction.
The base is not where I would like it to be. The speaker should go to 20Hz, maybe the amp can't handle it pushing the 3x 15" per Ch with the 15W per Ch. I have heard the speakers perform better. The Legacy Illusion speaker are pretty sensitive 98db , so should be okay I guess.
With the current setup I don't bi-amp and don't use the volume crossover control for the speaker may be that speaker sounds better with more amplification. I was just hoping for a more simple set up.
I have 3 Jadis integrateds, an Orchestra Reference, Defy DA30 (grandfather of the DA50S), and a DA60 (father of the DA88S). I own other amps, and have owned many amplifiers, both tube and solid state over the past 20 years. In my experience, Jadis amplifiers have proven more sensitive to tubes than any I’ve encountered.
Though the Defy DA30 / DA50S sits further up the ladder than the JOR, it does not produce as much power or bottom end. Art Dudley also mentions these in his review of the JOR. Your amplifier focuses on sublety, not brute force. In my experience, Legacy loudspeakers work best with higher output solid state amplification, and don’t couple well with low to moderately powered tube products. Sensitivity makes up one part of the loudspeaker - amplifier match. Impedance, phase angle, and crossover topology play an equally large factor. I’ve found my Defy 30 not at all a hot rod, and this match just doesn’t feel especially good to me. If you wish to hear what the Jadis can do, you may want to consider a more tube friendly loudspeaker, or moving on from the amplifier. The former should set the stage for you to begin to experience the beauty of Jadis.
Aside from that, personally, I would first try JJ E34L, JJ EL34, Shuguang EL34B, ElectroHarmonix 6CA7, Shuguang Black Treasure 6CA7-Z, or British made vintage Mullard EL34 output tubes, as I find Jadis integrateds to sound best with EL34. Outside of the Shuguang 12AX7B / Silver Dragon, most vintage 12AX7 better what recent amplifiers run, and that’s the next tube to change. If you don’t want to make much of an investment, but still hear the amp move forward, replace the drivers first, and keep the KT90 for now. Lastly, the 12AU7 input tubes, though I find the aforementioned tubes the first to address