Tube Amplifier Sound Characteristics

Amplifier sound boils down to topology and parts selection. The two extremes of topology will give either the most cold or most warm sound:
A. Push-pull design, class B, high feedback = "solid state sound", even when it is a tube amplifier.B. Single ended class A low or no feedback= "warm tube sound" (=live, as opposed to dead) with either tube or solid state.
Parts selection - a very long story, will not attempt it in a few lines.
However, it's the parts that will define how your chosen topology will sound like:So, the shapes that take form can be (from worst to best):A. the cold sound into any of these: boring, dead, clinical, mechanical, objective, studio.B. the warm into: syrupy, colored, subjective, natural, live performance, soulful, human touch.

The choice is: man or machine. Two different roads, both can attain heights or fall into a ditch.

Well I own Atma-sphere MA-1s and think they are among the finest sounding amps in the world. I dont even think about sonic characteristics when listening. I think OTLs are the best tube design provided certain speaker requirements are met.

The thing to ask yourself is why Atma-sphere gets so little attention from the audio press? I will tell you one thing it aint cause they dont sound wonderful. Truly in most cases the actual sound of a product may very low on the list of reviewer criteria. 

It surprises me that people still talk mainly in terms of tonal balance when discussing tube amps.
The warm, rolled off tube sound as it were.
Forget about it! It isn’t what you want anyway. Modern tube sound is better than that.
Tube sound is now about refinement, a sound that never fatigues or is edgy. This is the main reason I love the sound of tube amps. Too many solid state amps try to sound like tube amps by rolling off the highs and detail and do come off as slightly more refined- but boring!
Along with that refinement you get an open, clear sound and excellent transparency.
This is another reason I love the sound of modern tube amps- an open engaging neutral sound.
Last but not least the better tube amps deliver a stunningly spacious 3D sound stage. Solid state cannot touch this.
So kill the idea that the tube sound is about tonal balance, warmth and rolled off highs. That sound gets old in a hurry and it is time to be put out to pasture.  Little wonder solid state amps took off in droves.  
Tube sound is now about high end refinement, lack of any trace of harshness, leans more to a neutral beautiful open, transparent presence and midrange while presenting a spacious wide, deep and holographic sound stage.
The new tube sound should have solid state lovers coming back in droves. 

An n variable Radar chart, Just define your variables and the scales.

Ralph has it right, pick flavors your brain likes.

BTW the REF75 se is a superb amplifier, yes I have heard it length in my system….