Of the choices provided, my experience is that interconnects and speaker cables first, power cabling, and then fuses. I believe the answer is relative to the quality/performance of the item getting replaced. For example, going from 14 gauge Kimber cable speaker wire to Transparent Reference XL speaker cables was a dramatic change for me. Subsequent change unlikely to be that dramatic.
Similar to other posts, I believe there are things that take priority over the items mentioned.
Electric signal. I installed a separate meter connection for my electric, direct wired to audio room, with 6' deep copper pipe grounding and separate panel. If the electrical signal is nasty to start with, nothing (IMHO) can fix that. SQ will suffer.
Room treatment. Bottom line. If properly analyzed & installed, acoustic treatment will make any system sound significantly better, if not great.