I encourage trying tubes, signal first, then power.
You can plug ANY Line Level Device, Preamp, Processor, Line Controller .... into ANY Line Level Input of ANY Receiver/Integrated Amp/Selector, as long as you are careful with the multiple volume control’s relationship(s).
It can be a great way to get remote volume while using a vintage piece with no remote capability.
Your Yamaha unit has both ’CD Direct’ and ’Pure Direct’. I recommend you read the manual, I would try plugging a tube piece, or tube preamp’s output into the Yamaha’s ’CD Direct’ Input.
You will ’find’ and leave one unit’s volume at a pre-chosen position, and use the other unit’s remote volume control for more or less attenuation.
1. both/all unit’s volume down, connect preamp output to any line level input.
2. integrated/amp volume to 50%.
3. preamp volume slowly up.
Find a mix of the two volume controls for your default ’turn on’ listening volume.
IF you do low level listening, you want to use the Yamaha to lower the volume as it has a ’loudness’ volume feature that is beneficial, read your manual, it’s worth knowing how to properly use it. Maintaining proper bass relative to other frequencies is what keeps low volume music involving.
Select your source at the tube preamp, simply leave the Yamaha on the input the preamp is plugged into.
And, you can use the tube preamp for only a few sources, i.e. Phono and/or Tape out to the Yamaha’s CD Direct Input, and use other sources connected directly into another Yamaha input.
’Pure Direct’ on your Yamaha will bypass all the tone controls, probably bypass it’s balance control so if you use that, better to use the CD Direct.
..................................
In my case, I have a chain of 3 volume controls, one vintage tube tuner/preamp manual volume; one remote Line Controller with both remote volume and remote balance (why I use it); one Modern Integrated Amp with remote volume (but no balance control).
Sources to the tube preamp; Preamp to the Line Controller; Line Controller to the Integrated Amp’s default input (CD).
Preamp Vol at 12 o’clock; Integrated amp volume at 12 o’clock; use Line Controller’s remote volume and remote balance. I get it’s built-in automatic and progressive Fletcher Munson ’Loudness’ when I lower the volume, and at my chosen mix of positions I get more than enough volume when I raise the Line Controller’s volume.