KT150s in Audio Research Ref 110.

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. 

@samzx12 - I had a Primaluna Dialogue Premium power amp at the time and the 120s sounded too zippy, the top end was a little over emphasized. Originally I had a quad of SED winged C 6550s that I really loved, very balanced, easy to listen to, relaxed but detailed. When it came time to replace them they were considered NOS and no longer in production so I got the Sovtek 6550s and was very pleased. All of the tubes were bought from Kevin at Upscale. The Sovteks had enough of that SED sound to make me happy. After rolling through all those tubes over the years I just felt, from my experience, that Sovtek was overlooked. Of course we all know everyones ears, systems, and rooms are different! Hope this helps! 

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. 

This is from a well respected 30 year ARC dealer owner:

With matched tubes, all of which are well within the bias range of a Ref 110, it is no problem. Issues arise when people purchase 8 random tubes and “plug and play.”