Why do you want to do that?
It is always best to turn the preamp on first before the amp. That way everything can get settled in and there will be any pops, whooshes, etc...
I don't think you need a soft start- it doesn't matter to the tubes. It's a myth that it works for the small tubes we use. It applies to big tubes that operate in the KV region. Big kilowatt+ amps have a timed HV kick in relay.
