I own the HE-X4; my daughter absconded with them. Doubting I would ever hear them again I ordered the HE-5XX. Really love listening to them. With the HE-5XX and my Sennheiser 6XX I'm a very happy listener. I'm driving them with the Zen DAC Signature 6XX + Zen CAN Signature Amp 6xx (both from Drop.com). Much joy!
SoixThe HE-X4 sounds great. It was the sole reason I bought the HE-5XXs.
I got the HE-X4 on sale; I wouldn't hesitate to recommend them. They are a great value.
To (finially) answer you question, the HE-5XXs sound better to me. They sound bigger and brighter.
So for now, I'm not going to trade my daughter my HE-5XXs for the HE-X4s. 😉
DragonLanceThanks for your thoughts. I have the HE400S, which I’m guessing sounds more similar than not to the X4s, and the Arya Stealths but know very little about their 5-series ‘phones so much appreciate your impressions.