6550 vs KT88 vs KT90

From what I hear, these tubes are interchangable on most amps it seems that the 6550 is the least loved tube of the bunch. What are the sonic differences of these tubes? Thanks in advance for your reply.
Eldragon, I really love the Jadis. I have listened to many a product out there, and have EXTENSIVE experience with three in particular(Audio Research, Jolida, and Rogue). And, to me, the Jadis was a clear choice. I am somewhat surprised to hear the bass comments, because even when I had NO interest in buying a Jadis(late 80s - early 90s), the great bass(compared to other tube amps) was one of their main draws. I feel the bass is very good. Up there with any tube amp using 4 output tubes(can't compare to some of those BIG monsters using 16 or 32 tubes, obviously). And certainly, the best of ANY real world tube integrated. I believe it is a product of their massive transformers, large capacitance, and the KT90 tubes. It also has tone controls, so you CAN jack up the bass a bit, if ever needed. The amp is as detailed as I have been around. I hear things in music I have never heard before. CDs I have listened to hundreds(on various equipment) of times reveal new information. I am always amazed at cymbals, in particular. They are rendered with SUCH crispness, I always wonder why I haven't heard before. The detail may be a product of the passive preamp section. I like the combination of a passive preamp section into a tube integrated. The C - J CAV50 also does well using this combination. But, the Jadis is a MUCH better product. My main complaint with the C - J is its lack of power. Others complain about it being OVERLY warm, but I find that endearing of the C - J EL34 based amps. I imagine the Jadis could become quite warm with the Svetlana EL34s, if you are into that sound(I've yet to try).
Thanks Trelja, i have no reason not to believe you! It must be a great amp! Something i will seriously consider in the future! Regards!
I'll throw in my two cents, since I've got amps (Jadis JA 80's, vintage about 1990) which can use a few of the types mentioned. I agree with Greysquirrel that it depends on the amp involved. My amps won't run with the KT-90's (at least Frank Garbie said it would not be a good idea), and as between the KT 88s and 6550s, I prefer the former. The best 6550's I've used (old ARC Phillips and Svetlana 6550Cs; couldn't afford the Tung Sols, and the GE's just sounded ordinary) probably control the bass a little better, but ultimately sound leaner. On a rich balanced speaker (my old Duntech Princesses, for example), this was fine, but on a more neutral speaker they could sound too thin. The KT88s I use (old MO Valves) are much richer and more harmonically correct in the midrange, with good bass and a wonderful sweetness--the classic tube sound, I suppose. I'm curious as to how the new Russian KT88's stack up against the vintage tubes. My gut feeling is that the older Jadis amps were designed around the older KT88s; in fact, I believe the newer models may have been designed, particularly the transformers, around the KT90's. I've heard the newer Jadis amps with the KT90's, and they sound terrific. So I'd suggest you take the amplifier you want to use the tubes in into account. Hope I haven't confused you even more.......
Rcprince, you may want to e-mail Jadis regarding the use of KT90 tubes in your amp. They go out of their way to recommend KT90, and to avoid KT88 with me. They say they have seen reliability problems with KT88s. I have never had this experience. Quite the contrary. And, while Frank Garbie is very helpful, his information is often at odds with what I get from Jadis directly. So, again, I would ask THEM about KT90s in your amp. They always give me a response within a day or two. Good luck!
Trelja: Thanks, I'll give that a try. On the reliability, the newer KT88s, particularly the Chinese lower grade makes, have had reliability problems. The vintage stuff is a different story. All the 6550s I've used in the JA80s have started going bad, and blowing the protective fuses, within a year to 14 months (in my ARC Classic 150s, the same Phillips tubes lasted 5 years). I've had the KT88s in there for over two years now with no problems. I must admit having a preference for the MO Valve KT88s, even over the KT90s I've heard in Jadis equipment; just a matter of preferring the darker sound, I guess. But the prices.....