I would summarise it in a way to say there are three major aspects:
1. The amp electronics and settings
Given that guitar, cables, effects etc are optimised as far as you can, the best bet to work are settings. The impact of presence / resonance / gain are changing when you crank up the master. For myself I get along with the standard rule that, the louder you turn the (tube) amp, the lower the gain, eq, presence, etc should be to compensate for the increased impact of the power stage, speakers etc.
2. The physics of your rig
A combo like the VC30, but also any other combo or cab will start to resonate and rattle from a certain point on. There is a moment where - when turning louder - only the noise and boominess increases, but not the actual volume of usable guitar sound. Together with what's been said in 1., take it easy on bass and treble when you turn very loud, it's usually the mids that will make you louder in a band, not the bass. Also update your tubes regularly and either have a tech adjust bias from time to time (if your amp has adjustable bias) or get a Weber Bias Rite, which makes it fool proof to do it yourself. Tubes wear out and get worse at high volume when they're older.
3. The room
I'm sure you experienced it already yourself, wherever you place your rig, it will never sound the same. It's almost nerve wrecking to play the first few chords on a sound check, you never know before what you're dealing with. Some rooms sound great, some bad, some make your amp sound big and warm, others will enhance the spikey treble. Also if there are moving membranes in the room (e.g. other guitar cabs that are unused, drums etc) these will swallow lots of your sound. Sometimes an open back combo sounds bigger than a 4x12 if the back wall of the room is essential to shape a big sound etc. Check critically how the amp is positioned in the room, try to use corners or other elements to your advantage. Don't hide your amp behind a monitor speaker or sth. And adjust the EQ to the room. Being religious about those favourite settings won't help usually, adjustments need to be done to adapt to location.
