Maybe or maybe not a hazard; damage potential may indeed exist. It depends upon what the preamp does at its outputs when power is applied &/or removed. Some are safe & some are not; if it outputs any transient voltages then that would be coupled to the power amp & then amplified, possibly causing a blown amp fuse or even speaker damage. The safest way to find out is to use an oscilloscope to monitor the preamp's outputs, then turn power on & off while looking for DC component or significant (>100mV) transient activity. If you don't have access to a scope then a VOM might work, or else ask a dealer's service dept. to check this for you; it doesn't take very long & so shouldn't be very costly. Another way to test would be to wire up a cheap pair of speakers & live dangerously. Use some old 4X5 oval speakers etc, from junk.
mixing tubes and transistors
I'm considering getting a tube preamp in an otherwise all solid state system. The solid state gear is designed to be left on all the time. The tubes in the preamp wouldn't last very long if left powered up all the time. Here's my dilemma: the power amp owner's manual says to power down the components in sequence from the speakers "backward" to the source. The preamp is in the middle of this chain. If I turn off the preamp (to extend tube life) while leaving the amp and sources on (to keep the transistors thermo-electrically stable) am I creating any electrical hazards for my equipment? Please advise, and thanks.