advice for retubing of Jadis JA80

I have a pair of JA80, currently using GEC 6CA7 as a power tube. If i were to retube the power tube, eg: current product EL34, 6CA7, KT77, what are the factors i should look into, please advice.
And also what is the best power tube (tube model & brand to fix in a JA80, that would produce the best sound for it?
Any expert can provide some idea for small tubes(12ax7,12au7 with brands; Nos or current product) to change the tonality.
Thank you
it should be GE 6CA7 as a power tube,typing error.
I'm sure you'll get a lot of opinions. In mine, it depends on the age of your JA80s and your sonic preferences. Older models, before they started using KT90s, sounded best to my ears with Gold Lion/GEC KT88s, but I like a romantic, lush sound. Vintage EL34s (or, if you can find them, KT77s), like Mullards, sounded very good too, especially in the midrange, where they could outdo the KT88s, but they were not as strong in the bass as KT88s and did not last nearly as long in the amps as the KT88s (one year vs. 4-5 years, in my case). I'm convinced the the transformers in the older JA80s were optimized for the KT88 tubes, though I have absolutely no way to support that opinion. Not sure about current production, but I do recall being told that the transformers were optimized for the KT90s when Jadis started going to those tubes in the mid 90s.

With respect to input tubes, I found they make as big a sonic difference as the output tubes. My preferences were Telefunken ECC 802 and 803s, which in my JA80s provided the smoothest frequency response and lasted forever (for a bit less, the regular Tele 12AX7 and 12AU7 were also top notch), but for a punchier and slightly brighter sound Mullard 8136 and 8137 (I believe those were the numbers) were also excellent. EI tubes were similar to the Telefunkens, just not as refined or long-lasting. I used a National 12AU7 input tube for a while at the then-importer's suggestion, it was OK but couldn't match the Mullards or Teles.
Hi:Rcp, Thanks for your vaulable opinion.
My JA80 is an early 90's(~1990-1991) production. I bought it from a friend and was told that it has only been used for a short period of time.(less than 200 hours), The power tube that came with it is a stock tube(GE 6CA7), if i'm not wrong.
I listen more on classical, piano, vocal and jazz.
I'm thinking to change the power tube in order to find the best sonic to suit my taste.
For NOS power tube is very expensive now, so that i would think of getting a current porduct to subsititue with it.
If i consider to get other power tube, do i need to rebias?
I had a pair of that vintage. They came stock with GE 6550s at the time, which were serviceable but nothing special, and the worst of the types you could use in the amps (unless you scored some Tung Sols). Your tubes were probably substituted by your friend. I can't speak to modern tubes, I was not that impressed with them, particularly their longevity, so I gave up on them after a short try.

You do not need to rebias the amps if you change tubes, they are self-biasing.

I would try KT88s first, then EL34s, in your amps. For large scale classical you'll probably want the extra power and bass of the KT88s, for smaller scale material without deep bass the EL34s might be the ticket. 6550s will give better bass control, but I find most of them to lose the midrange magic that you pay for in a Jadis. Exception to that is NOS Tung Sol 6550s, which sound quite close to the KT88s. Perhaps some modern tubes would suffice for the start for you to see what you like, but I note that ultimately NOS pay for themselves in their longevity, as well as their great sound.
Defintiely go for cryoed tubes. tubemandotcom sells them