There’s no standard, that I’m aware of, for the level at which streaming services are sent. It’s the same with tv broadcasts too, so there’s often a volume difference between adverts and content, or from one channel to the next. I suspect you’ll find it’s a case of “it is what it is”.
You could try inputting to the other :5 and see if it makes a difference.
You’ll need to introduce a line level amplifier or mixer with some gain. A “passive” unit will not accomplish your goal. You can probably find a relatively inexpensive mixer that will suffice. Just make sure that it offers some “gain”. You want to be able to amplify the input. Most mixers will not amplify.
I’m not sure that you will ever be satisfied with the results because each music service and each album will be rendered at a slightly different level. Even the same recording session, when a record is mastered at a different time, will play at a slightly different level.
If level match is critical for you, you’ll need to include an audio compressor, however the compressor will diminish some of the audio quality that you are seeking by introducing the record player.
From other posts in the community, this looks like a chronic issue with Sonos where a line-in audio source is much quieter than a streaming source. Likely a contributing factor to why Autoplay has a volume option that is pre-set to 50.
Just ensure your preamp is set to “line”. You could go to the additional length to test your built-in preamp with a non-Sonos speaker and the same y-cable.
Adding another amplifier in series is not the best option and can damage the Play:5
