Haaaaah~ I guess somebody has to do this...
-Dead or alive
A high current is needed to blow a fuse. Now why is there a high current? Better pray that whichever fault condition that results in high current isn't permanent. Transformer primary side insulation getting punched through because of double voltage applied is a common failing mode.
-Fuse voltage rating
You need to get a fuse voltage rating equal to or higher than the AC voltage. Meaning you can use a 230V rated fuse to replace a 110V rated one. But not the other way round.
-Speaker power rating vs actual power consumption / Fuse current rating
"Z550 is also sold as 500w speakers". This is because it is bullshyt.
I also have this picture of a 200W computer speakers that is powered by USB. Actual power consumption is way lower.
Use back the same current rating unless you know what you're doing. So 3A in this case.
-Size of step-down transformer
Since that 500W is bull, means you don't need a "500W" transformer. I put inverted commas because transformers are rated in VA and not watts, but consumer step-down transformers are often rated in watts and they bullshytingly assume a power factor of 1 for their load. The typical power factor of "linear" power supplies like that in speakers is taken to be ~0.6 if unspecified. But this doesn't matter in this case, because you want a VA rating equal to the fuse and this value already takes power factor into consideration. So 120V * 3A = 360VA = 360W step-down transformer.
And THEN you still multiply that by two because China.
Not sure if it's difficult to get a 125V rated fuse here.
