You can monostrap stereo tube amplifiers if you are careful. But generally speaking they don't sound as good- less delicate, more heavy-handed.
So a Y adapter is used at the input and the outputs are wired in parallel. However, the 16 ohm tap becomes the 8 ohm output and the 8 ohm tap becomes the 4 ohm output.
So a Y adapter is used at the input and the outputs are wired in parallel. However, the 16 ohm tap becomes the 8 ohm output and the 8 ohm tap becomes the 4 ohm output.