You are going to have to spend a bundle on Czech KT90 tubes. Personally, I've never seen a Czech KT90...
On a more serious note, tubes are made in Slovakia, not the Czech Republic. Ei KT90 tubes were produced in the former Yugoslavia, prior to the 1998 NATO bombing campaign. Not sure how the KT90 of today sound, as I haven't heard any.
To my ear, the 6550 is one of the most "untubelike" tube I can think of. I really find them flat and dead sounding. I do agree about the lack of resolution in general, but that also sounds like the amp is not working properly. The first two questions I must ask are, did you measure the tubes, and did you rebias the amplifier? Without knowing the answer to either of these questions, you're wasting your time by trying to draw any conclusions.
You must listen to The Doctor's (Mechans) advice. Unless you bought a Jadis new, or can verify its history with absolute confidence, you simply must presume it has been modified in some way that will totally and completely destroy the sonics. Until and unless you send it in to be gone over, forget about evaluating anything in regards to what it does or does not do. Yes, this is not going to be cheap. The shipping alone is going to be a bear. But, these products have literally been savaged over the years, and must be put back the way they were originally. Forget about whether it looks bone stock or not, without the schematic and the ability to check the voltages throughout the unit, you'll never know.
Personally, The Doctor and I have found the JJ E34L sounds best in a Jadis. For more bass, try a JJ KT77, but you'll give up a bit of the best midrange around. If you must have powerful bass with the midrange magic of the E34L, roll the 12AX7 and 12AU7 tubes, the 12AX7 being most important.
Not sure what the shape of a tube has to do with anything beyond having them fit or not fit into a component - not an issue with a Defy 7.