2.5mm is fine, the electricity has no idea if it's going to a hifi or a fridgefreezer, it's the same electricity.
Once it arrives at the amp it's voltage is changed and its converted from AC to DC anyway so anything done to the electricity prior to the transformer is undone anyway. Having said that lots of people believe it makes a difference to their sound quality so all you've got to lose is a few quids worth of cable and a double socket, give it a go
Marantz CD63 KI Signature
Project Debut II
Pioneer A656 Reference
Epos M5s
Atacama Stands
Maplin speaker cable cos I can't hear a difference
Various interconnects as above