Welcome to TalkBass Danny.
Tough crowd here today / baptism by fire....
This is the spec that is throwing us off - especially the last "x4", so more information would be helpful. (i.e. what amp you're using)
The 450x2 part, and your comment that you're running in stereo mode is relevant (but not sure how the last x4 comes into play. )
Assuming this is a stereo amp, you are running an 8 ohm load on each of the 2 internal amps.
Typically, I would assume the 450 watts is spec'd is at 4 ohms, (for each amp) and likely closer to half of that at 8 ohms ~225 watts.
If those assumptions are correct, you shouldn't be at risk at pushing the cabs beyond their power rating. (as long as you don't drive the amp significantly beyond its limits.)
How / where you set the volume is a separate issue, and there is no correct answer we can give you for this - i.e. "7" - there are just too many variables, and each amp has a different "taper" to it's level controls.
You have to let your ears be your guide; obviously, no more than the volume required for the situation, and if you start pushing some "real volume" you have to be alert to when you start driving the amp/speakers beyond its limits.