I believe the ultrasonic and infrasonic behavior of a SE amp must be carefully examined. Excessive phase shift or ringing in these regions will surely sabotage the amplifier's potential for greatness. Bass transients will sound dull. Likewise, if there is ringing in the ultrasonic region, the amp will sound hard and 'hollow'. The proper selection of core material, core size, aspect ratio, and winding technique is far more critical in SE designs.
Poor choices lead to soft, lazy, unrefined sounding amplifiers.
The Tango line is unique in that it is the product of two decades of continual development. World-wide, there are thousands of SE amps with Tango outputs. Remember, I do not sell these transformers anymore. I just still love them. Of the available triodes, the 300B plays the most records with the greatest ease and the most refinement.