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.
