People like Atmasphere & Berning do that. And, back in the old days there was Harvey Rosenthal (?) who came up with the 1st zOTL amp (I think I'm remembering this correctly??)
No. Harvey Rosenberg had a contract for a while with David Berning (which never went anywhere), at or near that time he coined the 'ZOTL' term, which refers to David's amplifiers (which employ an unusual output transformer, one lacking the usual limitations of normal output transformers; IOW not an OTL, and a brilliant design regardless).
The output transformer is put in place between the tube power output stage & the speaker input to buffer the tube amp from the wild swings in the impedance & phase of the speaker.
On the primary side of the output transformer, the tube power output stage sees a constant impedance. By working into a constant impedance, there is optimum power transfer from the tube output stage into the output transformer primary windings load impedance. So, the waters (if you may) are calm/serene.
On the secondary windings side of the power output transformer, the waters are rather choppy due to the speaker impedance & phase variations vs. freq. Several output taps are provided to match the speaker impedance such that there is more optimum power transfer between the secondary windings & the speaker input.
This set of comments are incorrect. The transformer does not buffer impedance or phase at all. It *transforms* it (hence the term). So whatever swings of impedance seen in the load are translated to much higher impedances which are what the tubes see. If the impedance of the load is too low, the tubes will make distortion and less power; if too high the transformer will ring (distort). That is why taps are provided.
This is also why a lot of designers see the need for negative feedback, to tame the distortion of mismatched loads on the tubes due to impedance variation in the load, as well as the distortion of the transformer itself.
You can have impedance variations in the load and have it work perfectly fine with an amplifier that has no feedback; that is to say that feedback is not required for a neutral presentation on a speaker that has a variable impedance curve. It turns out that the ear has a tipping point where it will favor tonality due to distortion over actual frequency response variation (and FWIW, just look at the charts of any speaker- **no-one** in the world has actual flat frequency response from any speaker; this is why I see the Voltage Paradigm as an entirely failed concept, not the least of which it ignores human hearing rules). IOW its often far more important to have low distortion in many cases than perfect voltage response.
As Bifwynn found out, it is the interface between the amp and speaker that is far more important than the cost of either the amp or the speaker. When you understand the Voltage and Power Paradigm concepts essentially you take a lot of the guesswork out of matching amps and speakers.
Imagine the money saved if the industry actually talked openly about this!