KT150s in Audio Research Ref 110.

Moo foo how would you describe the differences between the 6550 and 120?

They said the plate current is to high for the power supply I believe.

Exactly what I needed to see. Thank you jea48.  The Ref110 sounds awesome with the KT120 but I have 4 KT150 and was thinking about buying 4 more if the amp could handle the KT150s. 

Those are some expensive tubes.

George I understand.  Sometimes you have to pay to play. Been there with a  few tube amps.  From my understanding ARC ran low on 6550 tubes and the later production runs of Ref 110s used the KT120s.  

Thank you Minorl. Very informative and exactly what I was wanting to hear. I would never use tubes not recommended by the manufacturer.  I'm really not into taking unnecessary chances with expensive equipment as some people evidently are as I saw a Ref 110 for sale with KT150s.  I thought maybe they know something I do not or taking chances.  Let's go with the latter lol. 
My KT120s came from Upscale Audio and they were already in the amp when I bought it. They have approximately 300 hours and sound very nice.  In another amp I despised the midrange of the 120 but in the Ref 110 much better. 

I also had a Primaluna Dialogue Premium with 120s and I agree they were waaay to zippy.  My favorite tubes were the Winged C EL34. I tried many but not the 6550. The Sovteks aren't that expensive so i may give them a whirl in the near future. The only downside is have to buy 8 and not 4 lol.