For me, a cable change on the he400 made a significant difference. It definitely changed the sound signature. I happen to like the change. Both my cables (he400&d2k) are just to thick, heavy and inflexible. I bought a cable made by Chris_himself and I'm really appreciating the physical differences. Chris's cables are featherlite and very flexible. Also I made mine shorter for convenience. Just makes for a more enjoyable experience all around IMO.
Well, yours was the jump from copper to silver, so, yes, that would make a huge sound signature change. I'm not sure I want to change the signature quite so severely as Silver. The Silver was more under consideration for HD650.