Invest in some less sensitive speakers if 9 o'clock is your max with a passive!!!
Seriously though, the tube pre would give an output impedance that is both low and constant relative to the passive and that may be an improvement in itself.
Now a variable gain pre as you describe would mean a voltage divider on each input in addition to the volume control. This arrangement will not achieve unity gain (as that requires a different circuit all together) and it may not be a good option sonically.
Decware makes a unity gain tube pre. This would be a good place to start