My Jadis Int amp. distorts in the upper mid-range

Newbee has made a good point: Does your amp distort at all sound levels or only when you crank it really up? I doubt if it is clipping though, because most Jadis gear clipps rather benignly. Have you checked your tubes, when you power up your amp? Do they all glow uniformly and evenly? This goes especially for the KT 90s? If it is the tubes, and that would be my first suspicion and I would not see the culprit visually, I'd start switching the ECC83s first. I'd stick with the KT90s for this amp and you'll find good replacements on Ebay at reasonable prices. For the input tubes, I'd borrow a working pair, just to find the possible culprit and then get a good military grade pair, tested for low noise and microphonics. Telefunkens sound wonderful with Jadis gear, but they are expensive and you must make sure that they have the typical diamond marking at the bottom. They are occasionally offered here on A. Tesla ECC803s are very nice as well and cost less. Mullards add a nice warmth to the sound, but they also cost and they are rare. Basically, I'd say, that for this amp you are well served with a good military grade ECC83 tube of Russian,(Sovtec) Jugoslav (EL) or Slovak (JJ) origin.

Newbee makes a very good point again. Please tell us about the rest of your system.
By the way, have you checked the soldering in your wires. Are your connections clean and tight? If not , this also can cause distortion, though probably not to such an extent, than you seem to be experiencing. But all the same, I'd check, if I were you.

So the culprit is either the Jadis or in the Meitner gear.
400 hours is nothing as far as tube life is concerned and in my experience the KT90s are very reliable. You also said nothing about your tubes not glowing right. For example bright fireworks in a KT90 at switch on, an abnormal brightness in the glow of them, or one of them being just dead below a sort of silvery veil on the glas. So I'm beginning to doubt, if the fault is in the tubes at all, but I'd still try a replacement set for good measure, which you could then keep for later use if the trouble is elsewhere. Do you have the chance to try another CD player? If the distortion stops, you have the culprit. If not, its the Jadis, unless your speaker cables somehow don't work well between the Jadis and your transducers. I'd try another set and listen, but I doubt, that the trouble comes from there. Good luck!

Pierre is absolutely right, heck, I should have thought of that.
Saffy, how is it going? Please report back to us!
Cheers,