5M power cable from fuse box to HiFi

I assume you are 220V so you need only 2 wires plus ground.  I would buy shielded SOOW cable.  10AWG plus ground.  The shield is to reduce noise transmission to your signal cables, not to protect the power cable.

I really don't think you will see any benefit using expensive wire.  at my house, power coming in has 4-5% thd.   You need lots of power available but it is up to the power supply in the AMP to clean it up.

Jerry

Yes, 220v, 3 wires (hot, neutral, ground)

I have a Transparent PIXL power conditioner (bought it used) which made a very noticeable improvement, so I have a feeling the power cord used will also matter, although the biggest improvement will probably come from using a dedicated fuse (no back noise from computers etc).

Dedicated wires may not make much difference.  You either get enough power during transients or you don't.  Like I mentioned, the power is coming in dirty to having a really high quality wire to preserve the dirty waveform doesn't help.  I'm not familiar with your power conditioner but it sounds like it is working for you.  

I'm not sure what european country uses a neutral wire.  I bought a european amp and it expects to see 240V+, 0V, and then a ground for protection only. 

Unless you have 4 wires, 2 hot, 1 neutral, 1 ground, I would suspect you have the 3 wire configuration I described above. 

BTW, the dirty power I'm describing at my home is supplied to my amp by a dedicated 10 awg 240V circuit.  Not sure it makes much difference but I know for sure I'm not short of power to my amp.  

Jerry

Cleaning up dirty power is the job of the power conditioner, the job of a good cable is to not decrease the quality of the power (signal) and prevent other nearby cables from getting noise from it.

With a perfect amplifier, or a perfect power conditioner, it should not matter, but previous tests at least in my system show that it does matter quite a lot.

Do you know the three things we often talk about for speaker cables: resistance, inductance, and capacitance? They all matter for power cables as well.