KT150 to KT120..or KT120 to KT150

@decooney No doubt that combo works well. Question is whether these amps were originally designed for KT120’s or KT150’s?...my money says KT120’s, which would lead to your observations.

@daveyf these are the QS Mono 120s are 120watt amps designed and sold with KT150s new as stock. KT120s are doable yet I cannot run KT88s or EL34s in these amps.  Will burn them up, according to the designer.  These have the big transformers and power caps in these particular amps, very transparent too.. They do run higher plate voltage in these. KT150s are super linear in these amps. I sort yearn for a throwback KT88 type sound every once in a while, so the KT120s put me back closer to that when I swap them in.  KT150s and KT120s only in these.  Already had quad spares thank goodness.    

Wonder how many amps out there that are running KT150's were actually originally designed to run on KT88's, then modded to run KT120's and finally KT150's? 

Interestingly, my old ARC amp running GE 6550's was super sweet sounding, the NOS GE 6550's were a superb sounding tube; but these days, I believe the newer KT150's and 170's are more resolving and offer better power in the bottom end.

@daveyf, I had a Dennis Had Inspire KT150 Hot Rod amp for a year. Was nice, fairly resolving, yet I needed more than 10wpc for my larger 92.5 db amt/ribbon speakers.  And was sure the 150 tube could do more with more transformer and plate voltage behind it.  Sure enough, two years later ended up with the QS Mono 120 with KT150s. Wow, much fuller, bolder midrange, super transparent, hearing deep in to the sound stage, across the front.  Now he and crew are fascinated with KT170s. A good sign. Glad to see other new tubes coming out.  Would be nice to see WE do something less exotic and more mainstream.  Meanwhile I'm going backwards landing on a blend of old/new sound. Nice. 

My ARC VT100 MKiii  sound really good with 6550 compared ARC Ref 75SE KT150

VT100 MKiii more meat and more like the actual sound

REF 75se, more transparent sound and more like flying on the air ,it is beautiful, just different style

I personally prefer the sound of VT100 a little bit more.

I do not know if I install 6550 in Ref 75se,the sound could be better.