Hey BJ,
Of course it's alright to leave it on and power up your rack with the Furman. I do that with my rack gear all the time, and not just the Preamps, but effects processors too. the only thing you DON'T want to do that with is any power amp.
For powering up your rack: Turn on your Furman power supply then, turn on the main power switch on your power amp. Let it warm up for at least 15 seconds, then flip the standby switch on the power amp.
To shut down your rack: Flip the Standby switch, then turn off the main power switch in the power amp, then turn of the whole rack with the Furman power switch.
Follow that procedure every time and you won't have any problems with your rack.
As for the cab sims on the back of the Classic, we're getting into a grey area here. It has two options: Closed back 4x12, and Open back 2x12.
(It's not as antiquated as you might think. The fact is; the MP-1 Classic was the last rack mounted preamp offering from ADA before they disappeared in 1994).
Some folks might like the simulated mic'ed cabs from the preamp direct to the board, but with the sims coming from the preamp itself, your limited to one or the other. You can't even turn them off to go direct that way. This is why I prefer to use the Microcab II. I'm speaking strictly for myself here. I like to use the Microcab II after the poweramp and before the Split-Stacks. The Microcab allows me to dial in the mic position relative to the center of the cone, and dial in the amount of low end Thump I want with it, plus it has a lot more cab simulations to choose from. Most of the time, I don't use any of the cab sims in a live situation, I just leave all the cab selection buttons in the out position and just go direct that way, and blend that in with the mic'ed cabs from the mixing desk. It's kind of a generic direct setting, but it works for me. I've never used the direct outs from any of my preamps into a board or DAW.
I like adding the character of the power amp in with the direct signal.
Harley