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Like Tpreaves said it's really up to you and your equipment. I use GL Kt-88 and winged "C" 6550. I rotate speakers and switch the tubes based on speakers. The 6550's have a little more punch on the bottom end and are a little fuller up high. Having said that it's based on my equipment, room, wire and ears.
If you have the first version of the Defy 7, that version could only be used with 6550s (though I suppose you could try to rebias them for other tubes, but that would be difficult), unlike the mono amps like the JA 80 and 200. I understand some later versions were able to use different tube types. My experience, with the JA80s (an auto-biasing design, so I could use KT88s, 6550s and EL34s withoput having to rebias), was that the 6550s had tighter bass but were a bit harmonically thin in the midrange and highs, overall sounded a little coarser in those regions than the KT88s; the KT88s gave up a little control in the deep bass, but had a much fuller, more correct midrange, and were perhaps a little less extended in the highs. Dynamically they were pretty similar; I guess my bottom line was that the 6550s were better if you wanted or needed more brute force, the KT88s were better when you wanted some refinement and finesse. But like the other two posters note, you should try for yourself and see which you like better, I'm sure you and I don't necessarily hear the same way, and I was comparing good 6550s (Philips, early Svetlana) with true NOS MO Valve KT88s, perhaps the best ever made of that type--it's possible that the differences in today's tubes are less marked.
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.
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.