Does anyone leave their amp and preamp on all the time?

While I don't leave my entire system on all the time, I use 833-A  single ended triodes to run the bass in a tri-wired speaker system, the cathode heater on an 833-A runs at 10 amps and 10 Volts and it will not let you get away with anything but the purest DC because it hums loudly otherwise. The power supplies for the heaters need at least 4.5 Farads from automotive digital capacitors to filter out the hum. But you can't hit them with the current from the 10 volt transformers, heavy duty rectifiers, and the 0.1 Ohm dropping resistor with such colossal capacitors emulating by force and effect a short circuit. Therefore the capacitors have to have a charge maintained by 9 volt AC to DC converters that maintain this charge through 100 Ohms before you start up the system. I am certain that there are other SETs that use a less exotic triode, but once you hear what it does to the bass you never want to go back to something more ordinary and less fun.

I turn off my tube units from the back and if I don't listen for weeks I unplug them off. 

So my last out of town trip, I turned off my DAC, preamp and amps off for two days. Sounded pretty hard for about 48 hours. 

This trip I was out longer, and turned off only the DAC. When I came back it seemed just fine immediately.

My preamp is a Parasound P7, and amps are Class D ICEpower ASP designs. 

So.... yes, it seems that somewhere in those two items are things that like to be left on. 

I have a VAC Auricle preamp (w/phono stage). I called VAC, and Brent, their head tech said tube wear is less by leaving it on all the time. The VAC monoblocks are turned on only when in use. 
For solid state gear, including power amps, it is the on/off power pulses that wear out components. Anything with a motor (CD players, open reel tape machines, etc.) should not be left on when not in use.
It has worked for me.

Preamp tube wear is less when left on? That is utterly positively and absolutely not true. Also, the myth that power amps suffer from on/off power pulses might be true for poorly designed gear, but I've owned lots of amps over 5 decades and that has never been an issue.