KT150 to KT120..or KT120 to KT150

@wolf_garcia
On that note, KT150s were not particularly special in my former SEP Class A DH Inspire Hot Rod amp either. Seems most like the smaller tubes with the smaller transformers in that amp. I concluded that KT150 tubes were a waste of $ and no value-add in those smaller footprint amps.

However, KT150s in my current Quicksilver Mono 120 tube amps is at an entirely different level of sound, stage, and enjoyment for sure. Transformers are 2.5x the size, higher plate voltage used, and yet I can manually bias them down to about 40ma per tube, and it sounds closer to something between EL34 and KT88s in triode mode in my prior Cary or Inspire amps. Owners stating something similar about the higher $ CJ amps specifically designed for KT150s.

Seems the specific amp design, the circuit, and use of sizable transformers sure make a difference for running KT150 tubes in their most optimum state. Now seeing more amps designed just for KT150s.  

As an owner of an amp designed for the KT150, the ARC Ref 150 SE, my only comment would be why even wade into the weeds as to which is better is any particular amp when New Sensor has terrible QC in general and most terribly with the KT150? For over two years now, New Sensor has failed to make any improvements in their QC while continuing to put out KT150's to the peril of the consumer. 
If you buy a quartet or octet of "matched" KT150's from virtually any source, you are doing so at your own risk. And in case I am not clear to you, I am saying that most vendors who claim to match tubes do not do so very reliably. They either lack the necessary equipment, the necessary know-how, or both. 
If your amp has fail safe features such that an arcing tube will not necessitate any repairs and your supplier is willing to give you 60 days, I could see venturing into the 150 waters. Otherwise, I would stay away unless you have money to burn AND don't mind the inconvenience of taking your amp in for repairs/sending it back to the manufacturer for repairs. 

The varying opinions of the KT150 tube seems to be primarily dependent on whether the amp was designed to utilize the KT150 in the first place. Since the KT150 needs a more stout transformer and has to run on a higher voltage, i would question if those who are not pleased with this tube are simply seeing the results of swapping in the KT150 into an inappropriate platform! 

@fsonicsmith   You are using the KT150's supplied from ARC? They do seem to have good QC, better than most. In the past, ARC has told us that rolling in tubes from other vendors is dicey, regardless of the tube type.

@fsonicsmith
Does your ARC 150 amp have the upgraded and lowered main circuit board mod for KT150s, and with more clearance for the top cover?

I remember reading some (not all) of the ARC tube amps around 2015 having issues with KT150s and main board changes, some failing resistors or something, among other things got redesigned. Bias issue and close matching tubes might not be as much of an issue after mod, I don't know. Also, comments about improved sonic quality and longevity.

Seeing redesigned amps by different brands with KT150s past two years, and suspecting maybe they figured out how to drive the tubes differently, fwiw. I wonder if ARC might offer any tweaks for them today?    

Found a helpful article for anyone comparing the jump from EL34 to KT150 output tube based amplifiers.    

My former amps were EL34 output tube based.  Liked them fairly well. Loved the midrange.  Now running other new mono tube amps, jumped through KT120s, on to KT150s, not letting myself forget the former, EL34s.. 

Initially, I missed my old amp and the magical EL34s, until the KT150s got past 150hrs burn-in. Also, found the right pair of small input/drive tubes to pair up, thus changing the sound of the KT150 result in a great way.  After burn-in, soon realized I did not miss the EL34s or KT120s any more.  

Article, Hifi-Advice:  
See the "From EL34 to KT150" or "From KT150 to EL34" paragraphs.  A helpful read to confirm what I've been hearing, even with completely different amps yet experiencing similar findings after making the jump.
https://www.hifi-advice.com/blog/review/analog-reviews/amplifier-reviews/primaluna-dialogue-premium-...