Getting back to last volume

Put a volume envelope on one of the tracks. Or just write a automation envelope.

Normally ctrl+z would work, but in this situation I want to lower multiple volumes, the record a track, then return the volumes to where they were. ctrl+z would only undo the recording. The reason I am doing it this way is a monitoring workaround.

you could in theory right click on the volume fader in track view or console view and select; VALUE-Set snap to current.
 
this way everytime you double click on the fader it will now snap to your new volume and not snap to zero like it is now.
 
in other words, normally double clicking on a fader snaps its default to 0db.
you can set that default value to where ever you want by the above example ive given.

Another way would be to lower the faders you want lowered - record your new track, then right click each fader you lowered and select "revert to [last stored value]"
 
Another way which I often use is simply to lower the level of the sub-busses all the other tracks are feeding - drums/guitars etc.
Using this method there should be far fewer buss faders than track faders to have to worry about. Also, I like to try and keep all of my busses at 0dB - so double clicking them gets them back exactly where they were.