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

Hi All
Just bought a Jadis Orchestra Refference Integrated amp. (used) - and it's distorting in the upper mid-range levels..

It uses 4x KT-90, and 2x ECC83 imput tubes, with about 400 hours on them - and were the ones originally supplied new from Jadis.

I have 100% elliminated my other components as a posible sourse of the distortion..

Please help !!

Also: If I have to change my tubes, does anyone have first hand experience at what tube upgrade is PERFECT for the Jadis Orchestra Reference Int. amp.

Thanks