KT150s in Audio Research Ref 110.

For several years I had auto-bias integrated and power amps and I went through all the power tubes (and several small pre tubes). I ran El34, kt77, kt120, kt150, Kt88, 6550. I kept coming back to the El34s and the 6550s. At the time I had a pair of Spendor sp2/3s that the Sovtek 6550s sounded wonderful with. The Sovteks were new production and reasonably priced. 

@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!