In any case 30 or 40VA is way over circuit needs.
12VAC @ 500mA means 6VA , anything else is (ample) safety margin.
Fuse rating is basically VA/mains_voltage , multiplied by a safety margin to avoid, as mentioned above, "nuisance" fuse blowing.
In such a case I'd be happy with 1.5X ; agree that on motors and such heavy loads up to 3X might be needed.
Fuse choice often is not meant to "protect the device" but bo protect home wiring, avoid fires, etc. , so current rating might not be chosen by device consumption which might be very low and ask for impossibly small fuses (just try to find a fuse for a 6VA 230V transformer which is what that device asks for) but simply way less than what the outlet can safely supply.
As in:
* actual device needs: (6VA/230V)*1.5= 0.04A
<-- try getting one 
* wall socket capacity : 10/15A?
* practical fuse choice: say 0.5A
will blow if transformer overheats and shorts, hopefully before it catches fire ; alternatively will blow before home main fuse (or tripping circuit breaker) to avoid you cursing and fumbling in the dark.
Now if you want to protect the device, given it's rated 500mA you may fit a *secondary* fuse rated around 600mA to 750mA , with 1A being still somewhat reasonable.
Remember that fuses *already* have some extra margin built-in so even a 500mA fuse should be enough.
As a side note, I somewhat doubt the "improvements" justifying the use of an "only 12V" powered "tube preamp" , suspecting it more of a glorified pilot lamp than anything else.