You ask a very good question. I am surprised no one has answered, yet.
Like you, I was curious about using a tubed Head amp, to do double duty, as a preamp. I`ll observe, with interest, as to what the responses will be.
I purchased a Jolida FX tubed DAC/Headphone amp. It was my introduction to tubes and it sure made a difference, sonically. What makes this unit versatile, is that it can be used as a passive preamp. The volume control provides attenuation from digital source to amplifier. The drawback, it`s output impedance is high, so there is a drop off in the low frequencies.
According to information available for the Valhalla, on Schiit`s website: it is designed to pull double duty as a pre, just as you desire. It has a low enough output impedance to drive a wider range of amplifiers, as well
If unsure, why not call customer service at both Mfr`s. and ask them?