(...) Would be nice to read micro’s technical explanation on this and his listening impressions would be welcome too...
In order to clarify the KT150 / KT120 / 6550 subject we must stay away of gossip and subjective generalizations, and look at tube specifications and curves.
1. Filament current 6550 1.6A +/-5% , KT120 1.7-1.95A, KT150 1.7-2A
Considering designer margins, the difference in power is very small, and in most cases will be marginal considering the whole transformer power.
2. Plate current versus grid 2 voltage - the KT150 needs a significantly lower negative voltage for the same plate current.
IMHO it is why cj and some other manufacturers openly say NO. As this curve varies from tube to tube we risk inserting a tube that is out of range of bias adjustment at any point, having excessive plate current, risking ruining the tube and the output transformer before the user notices it. In order to use KT150's safely the resistor in series with the adjustment potentiometer should be changed. Because we have success with one set of tubes does not assure the next one will work. Or even worse, during burn-in you risk going out of range if you start on the limit.
In my case I had to turn the KT150 pots of the LP275m's almost fully anti clockwise to get the correct plate current, but it worked perfect. One KT150 tube could not bias correctly in the Audio Research's.
3. If the amplifier complies with 2, then in all other characteristics, except dimensions, IMHO the KT150 is usually a direct replacement of the KT120, but with very similar although not equal curves. Its maximum specifications outperform the KT120 ones.
Jadis uses fixed bias , the cathode resistors must be changed when moving from KT150 to K120/6550 or vice /versa. Some amplifiers also need to change the power transformer to use KT150's, I do not know exactly what is the change in them.
It is late here, I will go on subjective opinions and a few facts on Jadis mk1/mk2 in another post - curious that the truth about them is different in french and english!