How many fuses in a McIntosh MC275?

Are you sure it is a 200mA? From what I see in the service menu, the main power fuse is 250V 4A slow-blow and two more inside for speakers output are 250V 2A slow-blow. You can download a service menu from here.

you can only prove hearing improvements on the blind a/b tests. blind tests is where all audi'o'philes fail to ID stock fuse from aftermarket and other silly unnecessary stuff.
as you know, fuses tend to blow and usually are all visible on the pcb or direct wiring. the improvement you've heard is possibly from the quality of the contact so all you need to do is to clean fuse terminals with qtip and contact cleaner.

I read the schematic and found that the two fuses inside are actually for filament heaters, not speaker outputs.

Nothing wrong with blind tests per se but it's certainly a mistake to think that if someone cannot hear a fuse or whatever in a blind test that the fuse or whatever doesn't work in other systems. There are many reasons why audiophiles get null or inconclusive results with some tweaks. You know what I'm talkin' about.

Marakenetz, do tell of your experiments with blind tests of standard and audiophile fuses. Of course it would be disingenuous to suggest someone's direct experience was to be false or untrue due to an improper scientific method if you had not rigorously proven the null hypothesis yourself (adjusting for clean contacts and such).