I think this is a very accurate assessment of the HD650. Especially this bit:
"I think the 650's bring more to the table in sound stage and clarity than they lose in bass compared to the best sounding bass headphones I own."
The only thing to be aware of for anyone going from a headphone that offers a deeper, more extended sub-bass, you'll will instantly notice the hd650 never seems to hit "as hard".
I'm sure not having this extension probably is what makes the mids and highs so luscious and fun. It's difficult for my ears to adjust to it's relaxed sound after fine-tuning my system for brighter headphones.
Nearly everyone claims tubes make the hd650 truly shine. I'm not sure if I'm ready to test this, but am curious if anyone who owns both a planar and the hd650 can comment on the system dependency and if the gap closes a bit driven synergistically?
Even with everything I've said, the hd650 is without a doubt, one of the best headphones that's been out as long as it has and remained relevant as long as it has for that very reason.
My main comparison is HE400's to the 650's. My highest power amp is a 1000mw E9 FiiO so I can't comment on the sound compared to something like a 6 watt lyr, but most of the difference I hear in sound is in the upper midrange. It's one of those things that on some tracks, it's different, but you can't put your finger on it. Other tracks, it's night and day.
Listening to Dave Brubeck Quartet's Take Five on the HE400's, the cymbal work the drummer is doing has an easily distinguishable ring. The ring is still there on the 650, but it's like a lower fidelity version of it. The best way I can sum it up is veiled. The 650's are well designed and Sennheiser has really squeezed every bit of performance out of voice coil dynamic drivers but it's not really fair to compare them to even an entry level planar like the 400's. The other area the 400's outperform is detail in the bass. It's hard to describe as well, these are all my subjective experiences, but listening to something like Bonnie Prince Billy "I see a darkness" the 400's reveal more details about the bass. It's not louder, it's not deeper per sey, but you get a sense of the air vibrating as a huge bass string moves back and forth, there is more detail. As a sometime bass player I have never heard the bass guitar on a recording have that quality it does when you are playing one. There are certain sounds, not the note but the thick string quickly vibrating back and forth, the way it makes the nut buzz a little, the higher harmonics even in the lowest notes from the wound metal strings, you hear it when you play a bass, you rarely hear it on a recording but the 400's brought that out easily.
The same songs on the 650 still sound terrific but it's more like a good headphone like the 990, and not like the god tier that are electrostatic and planar magnetic drivers. On the other hand the 650's have a warmth to them, and again a kind of thicker sound, party bolstered by the slight bump in midrange bass and the veiled mids, and this makes them VERY forgiving of harsh sound recordings in a track, and it hides the flaws where as the 400's amplify the flaws. Also in spite of the veiled but bright sound of the 400's, they sound a little cold. If these headphones were a color, 400's would be blue/white and 650's would be a warm sunset.
So I'm continuing to listen to the 650's tonight and I think they have loosened up some. I have about 8 hours break in on them. I'm listening to a 192kbps recording of The Shins Saint Simon and realized the cymbals sound like crap. I love good clean audio but I'm not a snob when it comes to how I listen. The majority of my collection is mp3 of various bitrates and I usually don't care much what bit rate it's recorded at. Yeah I can tell the difference, I just don't usually care. But the 400's and 650's and my new DAC have changed my minimum expected quality of a recording, and I'm sad to say, 192 doesn't cut it even for casual listening.
I did a blind listening test once some audiophile rag was staging to see if you could tell the difference in bitrates on a recording, and I could spot the difference between mid and high bitrate mp3, and uncompressed, but it was hard. I had to listen over and over and in the end, it wasn't compression artifacts or anything like that, it was the echo of the guitar. IE the soundstage was SLIGHTly different from one encoding to the next due to the rapid attack and decay of the guitar echoing in the soundspace it was recorded in.
This isn't like that, I don't need multiple listens and closing my eyes, it's like a big red gorrilla waving a flashing neon sign "Bad encoding!!!!".