Better sound from 6550 or kt88 tubes in Jadis amp?

In my opinion, the answer is, neither.

I have three Jadis amplifiers, all can take EL34/6CA7, KT77, 6550, KT88, and KT90. And, I have to say that I prefer EL34/6CA7 (Mullard, then JJ) best, followed by KT77 (only tried JJ). Sovtek 6550 were the worst I've tried, followed by Ei KT90, then EH 6550, then EH KT88.

Presuming you employ the right driver tubes, the bass and overall power will not be much different (I've even found JJ KT77 to have a more bass in my DA30 and DA60 than the EH and JJ 6550 and KT88s I've tried), but the midrange will be far and away better.

hmmmm.....
I have a very early Defy 7 (s/n 213). It was originally designed to use to use the EL34 tubes. It was modified in '91 to accept 6550 and KT88's.
I also have a Jadis Orchestra Reference that I am running Gold Lion reissues on.
The JOR sounds magical. I am still working on getting the Defy up to speed. I bought some reissue Gold Lion KT88 tubes for it and when I bought it, it had JJ KT88 tubes in it.

So, If I try so EL34/6CA7 or KT77 tubes, will I have any issues or are they pin compatible with KT88 tubes?

Anyone tube rolling a KT88 amp should check out the new Canada Fuller tubes. They sound a little bright right out of the box and need a full 100+ hours of break-in but then smooth out and are better than any other I have heard. Lots of slam, strong tight base, a clean relaxed midrange, and smooth extended highs. I recently went from Audio Note Conquest Silver 300B amps to Quicksilver V4s with these tubes and the Canada Fullers remind me of a 300B on steroids.