Tube amplifiers need output transformers because tubes have a high operating voltage and a high output impedance. I'm not a tube circuit expert by any means, but I think 3-6K ohms per tube is about what we're talking about, and the voltage level is hundreds of volts. Speakers, obviously, are 4-8 ohms nominally and need much lower voltages, so there's a mismatch between the voltage and current levels a tube output stage works at and what speakers want. The transformer steps down the voltage of the output stage to the voltage range speakers operate in. Put another way, the output transformer allows the tube output stage to be designed for optimal performance independently of the vastly different speaker load.
Some companies have designed tube amps without output transformers, and, not surprising, they tout that their amps sound better because they don't have output transformers. Unfortunately, those amps are finicky, expensive devices.
Output transformers typically have multiple taps for different speaker impedance ranges, for better matching of the load to the tube output stage. The matching effect of the transformer allows the output power to stay about the same into different taps, which is why amps with output transformers are often rated with a similar number of watts into various impedances. The question is, why do the transformers on your amps have multiple taps, since...
Transistor output stages are low output impedance designs. Good solid state amplifiers have an output impedance of 0.5 ohms or less, often much less. As a result, as the speaker's impedance varies the transistor output stage will try to maintain a constant output voltage, within the design's limits, which implies that as speaker impedance halves output power doubles - a very nice property to have. And it also means that transistor output stages do not need output transformers for matching the amplifier's output stage impedance to the speaker's impedance. You can put an output transformer on a solid state amp, but IMO it just gets in the way of what the transistor output stage can easily do on its own, with the right topology and power supply.
So, am I dissing your amp? Only as a discussion topic.
It's like a Porsche 911, in that the architecture seems silly, but a 911 is really a joy to drive anyway due to exquisite execution. I think the McIntosh amps are like that too.
