Mains

Power cables. Upgrade the sound with these expensive power cables. I don't know if it does, though.

Multiway powerblocks. Expensive versions of the £5 ones from high-street. Unsure if any improvement? Mains

Purifier.I believe this is the only worthwhile investment in clean mains. The two above I think don't improve sound, they are just hyped up to look better.

IEC (Mains) Used for higher-quality connections, usually poweramps & subwoofers- this is because IEC has a higher current ability. Again utter rubbish?