1 preamp out to a 2nd preamp in any harm?

(You can do it if you are careful with levels but.....)

Why?

I do the one preamp to another all the time.
I use one preamp to 'tube'ify' my digital sound, and that preamp feeds my main preamp.
I also have another preamp I use for the tubed phono section, and that preamp also feeds my main preamp.

Ny problems at all.
I would suggest playing with the combination of volume controls to find the best combination.
For the digital preamp, i find allowing my main one to be set to 1 o'clock and use the tube buffer preamp to adjust gain works best.
For the preamp with a phono section, i tend to max out that preamp volume, and keep it's gain set low. and use the volume on my regular preamp to adjust gain.

I have played with as many as three in line just to fool around. Two is no problem at all.

I want to run a mono signal through the one preamp to a solid state 300 watt monoblock and then to the sub and from there to all the woofers in my other speakers. With the other preamp I will send the signal through a tube amp to the mids and tweeters. I like the sound of solid state in the bass and tube in the mid/tweeter range. Just don't want to destroy anything in the process.