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?