Is it ok for tube amp directly to the outlet?

Directly to the wall should be the unanimous choice.
You already know the sound using your line conditioner.
Try the amps directly to the wall.You will be surprised.

Actually, I have a 80A line from the main panel to the subpanel and from the subpanel to 4 outlets in the room. All the gound lines from the outlets go to subpanel ground and the subpanel ground to the house's main ground.

Is the subpanel's ground line should be separate from the house's ground line and is this dangeous, violate the city's code if I separating them?

If subpanel ground shares with house's main gournd, is this called dedicated line or are they must be separate ground lines?

Thanks!

Sounds like you are where you would want to be on this wiring schema.