Preamp and power amp's basic specs lack of technical details have always been my beef too, but I do understand manufacturers know full well no matter what they include in the specs, the vast majority of their potential customers would not care, or understand the meaning of the details provided.
Suffice to say the often quoted 1 V preamp output by the likes of Onkyo and Yamaha, are not the "Maximum" output under a specified condition(example: unclipped, load impedance>X ohms etc.).
So that 1 V could have been based on say a standard input voltage of 200, or 200 mV etc..), or for an AVR it could have been based on the output level that would drive the power amp section to their rated output level into 8 ohms.
Either way, it typically does not mean 1 V is all the preamp could manage.
Also, for digital inputs, you may also see 2 V, such as Denon/Marant'z AVRs/AVPs and that 2 V level is obviously based on the DAC IC's output spec, as long as it does not exceed the limit of the volume control ICs.