-- The input impedance of your power amp.
-- The length of the cables that connect the preamp and power amp.
-- Whether the outputs of the tube preamp are capacitively coupled or
transformer coupled. (Or if it is one of
the very few that are direct coupled, e.g., those made by Atma-Sphere).
Generally speaking, though, I would consider using a tube preamp to drive both
a power amp and a pair of subs, at least one of which has an input impedance as
low as 12K, to be cause for concern. Especially if the tube preamp has
capacitively coupled outputs (as most tube preamps do), since the resulting
output impedance most likely rises considerably in the bottom octave or two
from what it is at higher frequencies. (The impedance of a capacitor
increases as frequency decreases).
Something you might consider, though, would be trying to find some sort of buffer stage, that would provide a high input impedance and a low output impedance, and inserting it in the path between the preamp and the subs. Or, if the main power amp also has a low input impedance, perhaps inserting it in the path between the preamp and both destinations. Something like the Burson Audio buffer stage that was marketed some years ago, but is no longer made.
Best regards,
-- Al