I'd use the current fuse and only upgrade it if it it blows.
My car has a similar system - Pioneer amp for sub plus separate stereo one for mains. The sub has "1200W" written on it which is likely "PMPO" so actually about 200W RMS. It has an 80 amp fuse and I confess to playing it loud at times.
Appropriate Fuse Size for Dual Amps
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