Your salesman and cleeds are correct. Don’t turn on the preamp with the amp already on. It “could” send a transient (thump or pop) to the speakers that can cause damage to them.
Not sure what preamp you have but working in audio manufacturing many years of my life it’s typically recommended to never turn your preamp or DAC off. Tubes take a minimum of 30 minutes or more to sound good. The actual power supply, depending on how much capacitance and regulation devices in it, can take even longer. Transistors can take much longer to stabilize. Even as much as 3 days or more. That’s why there is the standby circuit that is on all the time keeping a minimum amount of voltage circulating. Shouldn’t take very long to get back up to a good level of sound quality.
I now have a Schiit Yggdrasil that takes 3-4 days to sound great every time I unplug it. It’s thin, hard and bright sounding the first couple days and almost unlistenable. About the 3rd day it’s getting decent. After a week it’s really good. And that’s playing music through it 24/7.
Each piece can be different and you should be able to hear the differences as it warms up. It may only take 15 minutes. Then again with things like preamps or DAC’s it may takes days. I also have an old Theta tube preamp that has been highly modified all the way through. The way the power supply circuit works it multiplies the capacitance of each stage and I’ve added a lot in the PS so it literally can take a week or two to fully charge and sound its best. It all depends on the circuitry of your equipment.
It is also correct that turning things on and off often can be damaging to it. Think of a light bulb. It’s the turn on surge that blows it. Amplifiers take a lot of power so having a low power standby circuit is great. Preamps and DAC’s just leave on all the time as that don’t use much power. But then it’s up to you!