Absolutely fuse it within a couple of feet of the amp. An 80 amp fuse should do fine.
Why? Jl does not agree with you and they made the amp. How would having a fuse at the battery and a fuse at the amp make a difference?
Right from the manual for this amp right from JL's website
2) Run 4 AWG power wire from the battery
location to the amplifier mounting location.
Take care to route the wire in such a way that
it will not be damaged and will not interfere
with vehicle operation. Use 2 AWG or larger
power wire and a power distribution block if
additional amplifiers are being installed with
the XD500/3.
3) Connect power wire to the positive battery
post. Fuse the wire with an appropriate fuse
block (and connectors) within 18 inches (45
cm) wire length of the positive battery post.
This fuse is essential to protect the vehicle.
Do not install the fuse until the power wire
has been securely connected to the amplifier.
OP one fuse close to the battery is just fine.
