slow blow or fast acting fuse

They made a difference in my 35 year old Fisher 500 C.

It is not normally my practice to make generalizations, but a Sony receiver is not in the same class as a Fisher 500C. I have loads of restored vintage equipment, and a nice sounding old skool Marantz SR-19 AVR. Would I bother trying expensive HiFI tuning fuses in them ? No. IMO, the OP's money would be better spent having a tech give his Sony receiver a checkup. If bias or DC offset have drifted, adjusting them may prove more beneficial.

I looked at the fuse and there is no indication if
this fuse is fast blow or slow blow. I called and asked
sony and they said they are slow blow but I think they're
lying to me.

This one is for Gbart I happen to like sony es receivers
and I don't care for yamaha, nad, denon, etc. or any
other brand. The answer to Tls49's question is obviously
to get better sound.

Again, power supply fuses are almost always slo blow. Otherwise the turn on surge would blow them.

Speaker fuses are always fast blow. Otherwise it would take to long to effectively protect the speakers or the output stage.