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

Is this distortion at all levels? Could it be that you're just driving you amp into clipping? Does it appear in both chanels? You might have a mismatch with your speakers.....

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.

SAFFY UPDATE
The distortion is at low and medium levels - therefore clipping I believe is not the issue.
I have plenty of power and the speakers generally love tubes.

Saffy, i assume that the distortion is in both channels. If not, you could try swapping tubes around using a process of elimination til you find the bad tube. But if its in both channels i doubt that it would be one tube, either power or driver. Whats left to consider is the type of tube being used. Detlof has suggested something along this line and his recommended Yugo EI12ax7A elites have a very smooth clean mids and highs (i use these and highly recommend them). But, since i don't know what this intergrated amp replaced you may have just substituted in a product which is allowing you to hear your stream equipment and the fault may be there. By the way how did you reach the conclusion it wasn't your other equipment? A lot of folks discover distortions and frequency tilts when they plug in a higher resolution amp/preamp only to discover the source is a cd player or some such.

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.