I plug HE400 into Matrix mini-i for burn-in, after 100+ hours I've take listen for a while it's sounds not bad.
I think your may find amps that make HD650 sound good, and that will also nice with HE400.
Yes, power cord will make different for sure.
Nice PCs and bring wider soundstage and improved sonic, start at $300(list price) or pass entry level.
Problem is maybe these PCs will expansive than your amp, find...