Going back to autotransformers...presumably you would use them to make the 4 ohm MMG look like an 8 ohm load. If you had a solid state amplifier this would reduce the power delivery of the amp. (Macintosh used autotransformers the other way around in their ss amps so as to make 8 ohm loads look like 4). However, since you have a tube amp the output transformer already has a 4 ohm winding. Its 8 ohm output is just more windings in the transformer. If you used an autotransformer it would simply amount to replacing these windings with ones in an external device.
The most sensible application of these autotransformers would be as part of a crossover network so as to use less costly values of inductors and capacitors when the drivers are 4 ohms.
The most sensible application of these autotransformers would be as part of a crossover network so as to use less costly values of inductors and capacitors when the drivers are 4 ohms.