tubed preamp off and ss amp left on?

Why can't I leave my my ss amp turned on with the tubed preamp turned off? Loud hum occurs through speakers with this condition. When I had a ss preamp I could leave the amp powered all the time with the pre off. I would leave all powered up but concerned about the preamp tubes wearing prematurely. Is it a reasonable conclusion that I could leave everything powered on all the time and just change the tubes once per year?
Thanks for the responses. I just rechecked and the loud hum when pre is off and amp on is gone. Why? I do not know. I have made many changes lately trying to get rid of a low level hum in speakers when nothing is playing. This hum is still present. I have tried cheater plugs on two of the 3 components and the low level hum persists. Audible out to 2-3 ft from speakers. I have one more pair of interconnects to change. I doubt it helps but hopeful it will. If not I am going to change back to my old amp and see. BTW, the pre is lector zoe and amp is belles 350a reference. Other amp is aloia st15.01. What are the symptoms of an impedence mismatch? Can tubes create hum? I don't remember having this problem with stock tubes. May put them back in also. Thanks again.
Twc, if your cable tv is nearby, you should also try disconnecting the coax cable tv wire at its first point of entry and then turn your rig on and see if the hum is gone. If there's no more hum, you need a ground loop isolator which will take care of this issue for you.
Do you still have the hum when the preamp is disconnected entirely (with no cable attached to the amp inputs)? Are the pre and power-amps connected to the same outlet ? Just ruling out possible sources of ground loops.

also how old is the amp ? If it's old it could be failing power supply capacitors ? Just a wild guess.

The low frequency hum is almost certainly mains frequency (60Hz), so it's either coming through the amp's power supply, or via poor grounding somewhere.
Thanks. I will try disconnecting the preamp at the amp's inputs tomorrow as Mang53 suggested and see what happens. I hadn't thought of that. The amp is pretty new so I doubt it is the ps caps. I think I have had both pre and power pc's connected to the same dedicated outlet at some point but are on different dedicated outlets currently. No cable or sat. tv connected. Will post back tomorrow evening. Thanks again.